Spatially variant PMT cluster constitution and spatially variant PMT weights

ABSTRACT

A system including circuitry within a gamma camera system for generating spatially variant photomultiplier cluster based on a peak photomultiplier and for generating spatially variant weights for photomultipliers of the cluster depending on a cluster type signal. The system includes a first memory circuit addressed by a peak photomultiplier address signal and responsive thereto which generates (1) a unique photomultiplier cluster in geometry and size for the peak photomultiplier and (2) a cluster type. A resolution input signal (high/low) changes the size of the cluster. The first memory is programmable. A second memory responsive to photomultiplier addresses of the cluster and the cluster type, generates weight values for each photomultiplier. The second memory is programmable. The second memory allows weights for individual photomultipliers to be altered based on the geometry and size (e.g., type) of the cluster generated by the first memory. The system can effectively operate within a digital gamma camera system.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to the field of nuclear medicineinstrumentation. Specifically, the present invention relates toapparatus and methods for processing signals within a gamma camerasystem.

(2) Prior Art

Gamma camera systems contain one or more scintillation detectors eachcoupled to signal processing hardware and to an image generation systemthat may be one in the same. The scintillation detector is composed ofan array of photomultiplier tubes (PMTs) each generating a individualchannel signal that is responsive to light energy emitted byscintillations from within a crystal layer that is optically coupled tothe PMT array. The scintillations are responsive to interactions betweenthe crystal and a gamma event. The signal processing hardware typicallyprocesses the channel signal including multiplying the signal by apreamplification gain and also integrating the channel signal to arriveat an integrated result of an energy value.

The channel signals from a group of photomultipliers are then analyzedin a mathematical procedure to determine a spatial coordinate of theevent in two dimensional space (X, Y). The process to perform thespatial computation utilizes a group or cluster of PMTs that surround acentral PMT near which a gamma event occurred. In a centroidingprocedure, the coordinate of the gamma event is determined by analyzingthe energy signals of each PMT of the cluster and the total energy ofthe gamma event (as reported from all PMTs) using a weighted averageapproach. In the prior art, the constitution and geometry of thiscluster was fixed for all central PMTs. In other methods of the priorart, the cluster was defined by all PMTs having an energy signal over apredetermined threshold amount. Again cluster is fixed by the thresholdamount.

However, because of particular optical and geometric characteristics ofthe scintillation detector, use of a fixed geometric cluster is notadvantageous. For instance, a fixed geometric cluster does not offervariations in the resolution for a gamma event coordinate computation.Also, for central PMTs that lie on the edge or corner of a scintillationdetector, the fixed geometric cluster is often asymmetrical about thecentral PMT and this can lead to inaccurate spatial computations.Rather, what is needed is a gamma camera system that can define a uniquecluster constitution (e.g., geometry and number of PMTs included) foreach central PMT (e.g., the PMT having the largest energy response tothe gamma event). The present invention provides such advantageousresults.

Within the centroid computation procedure used to determine the spatialcoordinate of a gamma event, a weighted average procedure is utilizedbased on weight factors assigned to each PMT for the X and Ycoordinates. In the prior art systems, the weights (X and Y) assigned toeach PMT are fixed values. However, the PMT contribution to the centroidcomputation is not always a fixed factor. Other factors, such as thecrystal boundaries, optical interfaces, and PMT photocathode propertiescan make the PMT contribution different depending on the location of thegamma event and the position of the PMT with respect to the overall PMTcluster configuration. What is needed is a system that can providevariable X and Y weights to PMTs to permit higher accuracy centroidingand reduce demands on correction processing stages and also forpermitting a larger field of view of the camera system withoutincreasing the crystal dimensions. The present invention offers suchadvantageous functionality.

Accordingly, it is an object of the present invention to provide betterimage determination within a gamma camera system. It is yet anotherobject of the present invention to provide spatially variant clusterconstitution (configuration) to provide variable resolution within thegamma camera system and to provide more accurate determination of thespatial coordinates of gamma events. It is another object of the presentinvention to provide spatially variant weight assignments for PMTswithin a given cluster to more accurately determine the spatialcoordinates of gamma events. It is further an object of the presentinvention to determine the above variable weight assignment based on aparticular PMT's location and also based on a PMT cluster type. Theseand other objects of the present invention not specifically mentionedabove will become clear within discussions of the present inventionherein.

SUMMARY OF THE INVENTION

A system including circuitry within a gamma camera system for generatingspatially variant photomultiplier cluster based on a peakphotomultiplier and for generating spatially variant weights forphotomultipliers of the cluster depending on a cluster type signal isdescribed. The system includes a first memory circuit addressed by apeak photomultiplier address signal and responsive thereto whichgenerates (1) a unique photomultiplier cluster in geometry and size forthe peak photomultiplier and (2) a cluster type. A resolution inputsignal (high/low) changes the size the cluster. The first memory isprogrammable. A second memory responsive to photomultiplier addresses ofsaid cluster and the cluster type, generates weight values for eachphotomultiplier. The second memory is programmable. The second memoryallows weights for individual photomultipliers to be altered based onthe geometry and size (e.g., type) of the cluster generated by the firstmemory. The system can effectively operate within a digital garmnacamera system.

Specifically, embodiments of the present invention include in a gammacamera system having a scintillation detector for receiving gammaradiation, the scintillation detector having an array ofphotomultipliers wherein individual photomultipliers generate channelsignals, a device for generating a photomultiplier cluster, the deviceincluding: integration circuitry integrating the channel signals fromthe scintillation detector responsive to a gamma event and generatingintegration results therefrom; peak circuitry for determining a peakphotomultiplier based on the integration results and for generating asignal indicative of the peak photomultiplier; and cluster circuitryaddressed by the peak circuitry and responsive to the signal indicativeof the peak photomultiplier for generating a photomultiplier clusterassociated with the peak photomultiplier, the cluster circuitrycontaining separate photomultiplier clusters for individualphotomultipliers of the array. The present invention includes the aboveand wherein the cluster circuitry also generates a cluster type signalresponsive to the signal indicative of the peak photomultiplier whereinthe cluster type signal indicates a geometric configuration of thephotomultiplier cluster including a normal cluster type, an edge clustertype and a corner cluster type of the photomultiplier cluster.

Embodiments of the present invention include in a gamma camera systemhaving a scintillation detector for receiving gamma radiation, thescintillation detector having an array of photomultipliers whereinindividual photomultipliers generate channel signals, a device forgenerating spatially dependent weight factors, the device including:integration circuitry integrating the channel signals from thescintillation detector responsive to a gamma event and generatingintegration results therefrom; cluster circuitry responsive to theintegration results for generating signals indicative ofphotomultipliers within a photomultiplier cluster defined by the gammaevent; circuitry for generating a type value signal indicative of a typeof the photomultiplier cluster; and weight circuitry responsive to (1)the signals indicative of photomultipliers within the photomultipliercluster and (2) the type value signal, for generating individualcoordinate weight value signals associated with individualphotomultipliers of the photomultiplier cluster. The present inventionincludes the above and wherein the coordinate weight value signalsassociated with individual photomultipliers of the photomultipliercluster represent an X axis weight value and a Y axis weight value.

The present invention includes the above and wherein the cluster typesignal indicates a geometric configuration of the photomultipliercluster including a normal cluster type, an edge cluster type and acorner cluster type of the photomultiplier cluster. The presentinvention also includes methods for use implemented in accordance withthe above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a high level block diagram of the gamma cameradetector, information processor and user interface device of the systemof the present invention.

FIG. 2A is a circuit block diagram illustrating elements of the triggergeneration logic of the present invention.

FIG. 2B is a circuit diagram of the circuitry utilized by the presentinvention for generating an analog total ("global") energy signal.

FIG. 2C illustrates the logic of the present invention (utilizing dualintegrators per channel) for providing a compensated digital responsesignal that is digitally integrated for each PMT of the gamma detector.

FIG. 2D illustrates the processing blocks of the present inventionDigital Event Processor for generation of the spatial coordinates of anevent, local and global energy, peak PMT address and the associatedsignal output.

FIG. 3 is an illustration of a weighted centroid computation fordetermining the spatial coordinates of an event used by the presentinvention.

FIG. 4A and FIG. 4B illustrate an overall flow diagram of aspects of theDigital Event Processor of the present invention.

FIG. 5 is a diagram of a digital processor (computer system) and userinterface of the present invention.

FIG. 6A represents the light intensity response for multiple eventsoccurring close in time within the present invention.

FIG. 6B illustrates a flow chart of the timing performed for dualintegration on a particular channel.

FIG. 7 illustrates an exemplary PMT array of a detector of the presentinvention and illustrates three different cluster types.

FIG. 8 is an illustration of the PMT address table circuit of thepresent invention Digital Event Processor.

FIG. 9 is an illustration of the x and y weight table circuit of thepresent invention Digital Event Processor.

FIG. 10 illustrates a plane view of a collimator (with edges) aspositioned over a PMT array of the present invention.

FIG. 11 illustrates placement of centrally located PMTs and obscuredPMTs and an exemplary strip area associated with an obscured PMT.

FIG. 12 illustrates energy absorption percentages for a typical PMTcentroid arrangement for a gamma event.

FIG. 13A is a flow chart illustrating initial calibration tasks executedby the autogain procedure of the present invention as typically executedat the factory during calibration or in the field without a collimator.

FIG. 13B is a flow chart illustrating routine calibration tasks executedby the autogain procedure of the present invention as typically executedat the site during routine calibration with the collimator removed.

FIG. 13C is a flow chart illustrating routine calibration tasks executedby the autogain procedure of the present invention as typically executedat the site during routine calibration with the collimator installed.

FIG. 14A illustrates an uncompressed response of a PMT for an eventverses the distance from the center of the event and also showscompressed response.

FIG. 14B illustrates a summation of PMT responses (of a PMT cluster)over a one dimensional PMT configuration.

FIG. 15A and FIG. 15B and FIG. 15C illustrate exemplary compressionprocedures realized by the dynamic compression table and circuitry ofthe present invention.

FIG. 16 illustrates an analog voltage wave form (with baseline offset)of a preamplification channel of the present invention.

FIG. 17A illustrates a flow chart of the automatic baseline offsetcalibration of the present invention.

FIG. 17B illustrates a flow chart of the software triggered baselineoffset correction procedure of the present invention.

FIG. 17C and FIG. 17D illustrate flow charts of the scintillationtriggered baseline offset correction procedure of the present invention.

FIG. 18 illustrates the memory circuit of the present invention forproviding far PMT addresses based on an input peak PMT address.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes an apparatus and method for improvingaccuracy of the spatial coordinate determination of gamma events byproviding spatially variant PMT cluster constitution based on a coarselocation determination of the gamma events. The coarse location isdetermined based on the peak PMT for the gamma events. The presentinvention also provides spatially variant centroid weight determinationfor X and Y coordinate for each PMT. The weight assigned to a particularPMT in the centroid computation is based on the spatial location of thePMT and also the PMT cluster type.

In the following detailed description of the present invention numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be obvious toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances well known methods,procedures, components, and circuits have not been described in detailas not to unnecessarily obscure the present invention. Some portions ofthe detailed descriptions which follow are presented in terms ofalgorithms and symbolic representations of operations on data bitswithin a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. Unless specifically stated otherwise asapparent from the following discussions, it is appreciated thatthroughout the present invention, discussions utilizing terms such as"processing" or "computing" or "calculating" or "determining" or"displaying" or the like, refer to the action and processes of acomputer system, or similar electronic computing device, thatmanipulates and transforms data represented as physical (electronic)quantities within the computer system's registers and memories intoother data similarly represented as physical quantities within thecomputer system memories or registers or other such information storage,transmission or display devices.

The various embodiments of the present invention described herein areused in conjunction with a scintillation detector of a gamma camera.With reference to FIG. 1, a high level diagram of a gamma camera systemis shown. Generally, the system of the present invention includes gammacamera detector 80 composed of a plurality of photomultiplier tubes(e.g., 17, 6, 0, 2, 9), PMTs, arranged in a two dimensional matrixconvention and optically coupled to a glass plate to receive light(e.g., visible photons) from a crystal layer 81. The PMT array creates aphotodetector. The crystal layer can be composed of sodium iodine, NaI,and is typically located between a collimator 83 and the PMT array. Thecollimator 83, as is known, is typically manufactured from a number ofholes with lead septas arranged in a honeycomb convention to collimategamma rays that strike the crystal 81.

Gamma rays that strike the NaI(TI) crystal 81 cause well knownscintillation events that release a number of visible light photons thatare detected by the PMTs with different light intensities. Each PMTreports in the form of an analog signal indicative of the amount oflight energy detected as a result of the scintillation event. The gammacamera detector 80 utilized within the scope of the present invention isof the Anger type and can be of a number of well known and commerciallyavailable designs and therefore the details of such a gamma detectorwill not be discussed in depth herein. An exemplary gamma cameradetector used by one embodiment of the present invention can contain asmany as 55 or 108 PMTs. The detector 80 can also utilize smallerdiameter PMTs along the edges to increase the detector's field of view.An embodiment of the present invention utilizes forty-nine 76 mm roundPMTs and six 51 mm round PMTs for edge filling, however, the number ofPMTs, their sizes and their configurations can be varied within thescope of the present invention.

The detector 80 of FIG. 1 is mounted on a gantry 85 which can rotate thedetector 80 in various orbits around an object (patient) resting ontable 87 (e.g., for ECT scanning operations). The gantry and table reston base 89. The detector 80 may also be directed transversely across thetable 87 (e.g., for total body scanning operations) or placed over thepatient for static imaging.

The analog output signals from each of the 55 PMTs are output to aninformation processor 91 that includes a general purpose digitalcomputer system as described to follow. The processor 91 containscircuitry for adjusting and compensating the signals received from eachPMT and also for digitizing these signals. Utilizing techniques as willbe further discussed to follow, processor 91 computes, among othervalues, the spatial coordinates (X, Y) and energy coordinate, Z, foreach gamma event. Imaging information detected by the gamma detectorover sampling periods is then stored in digital storage memory (e.g.,counts per coordinate) and is visualized on a monitor (or a hardcopydevice) of a user interface device 93 in response to commands from theuser interface device 93.

INFORMATION PROCESSOR

The circuitry and logic of information processing unit 91 of the presentinvention is further illustrated within the following figures: FIG. 2A,FIG. 2B, FIG. 2C, and FIG. 2D. It is appreciated circuitry of the abovecan alternatively be located within the detector head 80 itself orspread among both the detector head 80 and the processor 91. It isfurther appreciated that the processor 91 also comprises a generalpurpose computer system 1112 (see FIG. 5) as will be discussed in moredetail. An embodiment of the present invention can also be implementedutilizing discrete electronic components in lieu of a general purposecomputer system or, alternatively, a general purpose computer system canbe used.

TRIGGER DETECTION

Regarding FIG. 2A, the trigger signal generation circuit 100 of thepresent invention is illustrated. According to the arrangement of thePMTs in the detector, the output of each PMT (compensated as shownherein) associated with a particular spatial quadrant (or "zone") of thedetector matrix is sent to one of four trigger detection circuits 110a,110b, 110c or 110d. Although the precise alignment of each PMT in eachzone is not critical to the present invention, the zones areoverlapping. The PMT signals A0-A54 are voltage signals. The signals ofthe first zone, A0 . . . A17 are sent to trigger circuit 110a where eachis coupled to an inductor 102 and an amplifier 104 which together createa 200 ns clip circuit.

The clipped signal from circuit 104 is coupled to the positive end of adiscriminator circuit 106 and a computer controlled reference input iscoupled to a threshold input circuit 108. The reference signal at 108 iscoupled to receive the output of a computer controlled DAC (not shown).Therefore, only trigger signals over the threshold voltage are allowedto pass through circuit 106 and they are clipped to 200 ns. The outputof the comparator 106 is then coupled to the input of OR gate 120 vialine 122. Line 130 will assert a triggering pulse when ever a PMT of thedesignated zone detects an event. This circuitry is replicated for eachof the other four zones of the detector PMT matrix (e.g., signals A11 .. . A30 feed circuit 110b which generates a trigger over line 124,signals A24 . . . A43 feed circuit 110c which generates a trigger overline 126, and signals A37 . . . A54 feed circuit 110d which generates atrigger over 128). It is appreciated that additional or fewer eventdetection circuits (e.g., for more zones) can be used within the scopeof the present invention. Multiple trigger channels are used to maintaina high signal to noise ratio in the presence of correlated noise. Thetrigger channels are overlapped, as discussed above, to preventsensitivity loss at the zone boundaries.

Trigger input lines 122, 124, 126, and 128 of FIG. 2A are coupled to theinput of OR gate 120. Therefore, when an event is detected by the cameradetector 80, OR gate 120 of trigger circuit 110 of the present inventionwill generate a trigger pulse over line 130. These triggering pulses arecalled Start(t0) and Start(t1) and are used by the integration circuits280(0)-280(54) (FIG. 2C) of the present invention to start integrationof the PMT signals for the detected gamma event.

ANALOG SUM OF GLOBAL ENERGY

Refer to FIG. 2B which illustrates an analog summing circuit utilized byone embodiment of the present invention. The voltage signals A0 . . .A54 from each PMT channel (e.g., signal from each PMT) are summedtogether and output by amplifier 142. An offset voltage is fed into thesumming circuit via input 144. The offset voltage is controlled by acomputer controlled DAC. The output, or total energy of the gamma event,is generated over line A55. This output represents the analog sum of allof the voltage signals from each PMT channel plus an adjustable offsetfrom circuit 144. The analog signal over A55 is called the analog globalenergy signal. This global energy signal may be supplied to an availablechannel (e.g., channel 55) for preamplification, digitization andintegration by circuit 200 (FIG. 2C). The Digital Event Processor of thepresent invention receives signal A55. As will be discussed to follow,the global energy value may also be computed digitally by summing thedigitized integrated channel signals of the PMTs. Either of thesemethods may be utilized within the scope of the present invention.

PREAMPLIFICATION DIGITIZER

Refer to FIG. 2C which illustrates the preamplification digitizercircuits 200 of the present invention for each of the 55 channels (plusone for A55). These circuits perform the preamplification, digitization,and integration for each analog voltage signal for a PMT channel. Aswill be discussed, each preamplification digitizer circuit for eachchannel contains two separate integrator circuits. Circuit 280(0)corresponds to the current output signal received directly from PMT #0(e.g., channel 0) and this circuit 280(0) is separately replicated foreach of the 55 PMT channels of the present invention and, as shown,circuits 280(0) to 280(54) operate to simultaneously process the currentoutput signals for PMT0 to PMT54. Regarding circuit 280(0), the currentsignal output of PMT0 is fed into a current to voltage converter 210 andthe output of this signal is fed through a resistor to a voltage gainamplifier 222.

A computer controlled digital to analog converter (DAC) outputs twoadjustment signals over line 212 and 214 for baseline voltagecorrection. The signal over line 212 is fed through a resistor 216 forcoarse adjustment and line 214 is fed through resistor 218, which hasmuch larger resistance (e.g., on the order of 200×) than resistor 216,for fine adjustment. The signals received via lines 212 and 214 providea baseline offset voltage adjustment to the output signal received fromthe PMT0. A computer controlled digital to analog converter (DAC)outputs a voltage adjustment signal to circuit 220 to control the gainof amplifier 222 having an exemplary gain adjustment of 10:1. The analoggain adjustments are coarse adjustment with fine gain adjustmentsperformed by the calibration table, see FIG. 2D as discussed to follow.The baseline offset adjusted signal and the gain adjusted signal is thenoutput at point 224 as signal A0 for each channel. Similarly, for eachPMT channel, the above circuitry is replicated for generating signals A1to A54. The trigger signal 130 is uniformly supplied to each of thecircuits 280(0) to 280(54) so that each channel is triggeredcoincidentally.

The output of gain amplifier 222 is then fed into resistor 226. Thevoltage input 228 and voltage input 230 are coupled through respectiveresistors to the output of resistor 226. The output of resistor 226 isthen fed into amplifier 232 and capacitor 234 in series. The outputs ofthe amplifier 232 and capacitor 234 are then coupled to the input of theanalog to digital converter (ADC) 236. The above circuit (e.g., frompoint 224 to the input of ADC 236) is utilized for pulse insertion usedfor diagnostic purposes that are not particularly pertinent to thepresent invention. Pulses may be artificially inserted via inputs 228and 230.

Referring to FIG. 2C, the ADC 236 converts the analog signal A0 todigital samples based on the frequency of clock input as shown. Oneembodiment of the present invention utilizes a sample frequency of 25MHz as a sample clock. The output of ADC 236 is then fed to the input oftwo adders 238 and 240 coupled in an integration configuration. Thepresent invention utilizes dual digital integrators for each PMT channelin order to more effectively process conditions wherein two gamma eventsare detected in close temporal proximity. Each integrator 238 and 240contains a separate register (accumulator) for containing the currentsum value and performs piecewise linear integration with a clip binaryvalue at 1023 (e.g., no rollover is allowed). The output of bothintegrators is coupled to multiplexer 241. The multiplexer 241 selectsbetween one of the two integrator registers (accumulators) for output tothe latch circuits 242 and 244. Latch circuits 242 and 244 comprise atwo stage FIFO arrangement. The trigger pulses Start(t0) and Start(t1)received over 130 are used to reset the integrators. At the end of anintegration period, the value of the integration process for eitherintegrator is then stored within a two stage latch circuit of 242 or244. The output of the latch circuit 242 or 244 is the digitized valueof the signal generated by PMT0 for a given event and this value isdesignated as D0. The digitized signal DO is supplied to the DigitalEvent Processor 300. The above is explained in more detail withreference to FIG. 6B.

Using the dual integrators of FIG. 2C of the present invention, eitherone or both of the accumulators of 238 or 240 can be enabled tointegrate an incoming signal from the output of ADC 236. The twointegration results are multiplexed onto a common data path and eitherresult can be selected and stored in the two stage latch circuit. Eachintegrator may be separately triggered by start(t0) and start(t1). Inoperation, when a trigger signal occurs, if either integrator isavailable (e.g., not integrating and not holding an integrated result)then that accumulator is reset and enabled to begin integrating theevent. When either of the accumulators completes, the integrated valueis transferred to the first FIFO stage (e.g., latch 242) assuming thisstage is available. If it is not available, the accumulator holds thevalue. Integration continues for a predetermined period of time (thecamera's dead time) after the trigger signal until a sufficient amountof the gamma event's energy is integrated. Values are transferred fromFIFO stage 1 (latch 242) to FIFO stage 2 (e.g., latch 244) as FIFO stage2 becomes available (e.g., transfer its value). When data is written toFIFO stage 2, the present invention signals that data is ready to betransferred to the Digital Event Processor (DEP) 300, see FIG. 2D.

Referring to FIG. 2C, the dual accumulator design of the presentinvention provides an implicit mechanism for handling event pile-upwhere two events interact during the same time period. Since the presentinvention computes positions based on local PMT clusters, pile-up eventswhich occur in different regions of the detector can be properlypositioned and such are called temporal pile-ups. If the two events thatare involved in a temporal pile-up happen to be separated by more thanthe trigger channel deadtime, then both accumulators will be enabled andboth events will be fully integrated. Accuracy of positions will beimpacted by the spatial distance separating the events. The greater theseparation, the lower the impact. This is discussed in more depth below.

The circuit 280(0) is replicated for each channel as shown in FIG. 2C.The output current signals from PMT#1 through PMT#54 are fed intocircuits 280(1) through 280(54). The digital data signals DO to D54 areoutput from circuits 280(0) through 280(54), respectively. Each of the55preamplification digitizer circuits 280(0)-280(54) are coupled toreceive two trigger signals start(t0) and start(t1) from line 130. It isappreciated that an extra channel (e.g., a preamplification digitizercircuit 280(55)) may be added in order to process the analog globalenergy signal from A55 (see FIG. 2B). In this embodiment, the output D55would correspond to the amplified, digitized and integrated value forall channels (e.g., the digitized value of the analog global energy ofthe event). In such an embodiment, the value D55 would be output to theDEP 300 (as will be discussed to follow) with an appropriate PMT addressvalue indicating the data as analog global energy data.

The preamplification circuits of FIG. 2C can be directly adjusted usinggain (e.g., line 220) and baseline offset (e.g., lines 212 and 214)adjustments. The above adjustment lines are referred to as control linksignals which are coupled to computer controlled addressable DACs. Theamount of nominal baseline and the amount of variation associated withlevels of adjustment can be used to determine the accuracy of a givenchannel.

At the completion of an integration period, when latch 244 data ispresent, then all of the digital data stored in each second stage latchfor each channel is transferred to the DEP 300 (FIG. 2D) over bus 307.This is called a data "transfer" to the DEP 300.

DIGITAL EVENT PROCESSOR

Refer to FIG. 2D which illustrates circuitry 300 of the Digital EventProcessor (DEP) of the present invention. The digitized and integratedsignal values for the PMT channels over lines DO through D54 are fedover bus 307 to FIFO 310 and to calibration table 315. The data over bus307 represents the digitized integrated signals supplied from each PMTchannel circuit, 280(i), in response to a triggering event such as ascintillation event. The digitized data over bus 307 is stored into araw view FIFO 310 that can be accessed over bus 397 by a digitalprocessor computer 1112. The raw view FIFO allows data to be pulled fromthe input data stream without interrupting the normal data flow from thePMTs. This is utilized for on-the-fly baseline adjustment. A calibrationtable 315 receives as inputs (1) the digitized integrated channel datafrom bus 307 and also receives (2) PMT address (e.g., indicator) numberover bus 302 of the reporting PMTs in order to correlate the digitaldata over bus 307 with the proper PMT channel output from circuits280(i).

The calibration table 315 contains a lookup table (LUT) for providing again output which can vary with the PMT number input (represented in oneembodiment as an address); this spatially variant gain is applied to theintegrated signal value received over bus 307 and the result is outputover bus 347 which is a corrected or finely compensated integratedsignal value for each PMT channel. The gain value stored in thecalibration table 315 is a fine gain adjustment dependent on the PMTnumber whereas the gain amplifier 222 is a coarse gain adjustment. Thecalibration table 315 also provides a baseline adjustment computation bydigitally subtracting the reference voltage inserted by the baselineoffset circuitry (e.g., inputs 212 and 214) of the preamplificationcircuits for each PMT channel independently. The output over bus 347 isthe digitized values supplied from 307 with this baseline adjustment.

The output of the calibration table 315 is transferred over bus 347 to apeak detect circuit 320 which analyzes all the calibrated results of all55 channels (for a given data transfer) and selects the PMT numberhaving the largest integrated channel signal (e.g., "energy") output fora given measured event; this is the "peak PMT." The maximum integratedsignal value and the associated channel address are retained for lateruse in the DEP process of the present invention. The integrated signalassociated with the peak PMT is output from peak detect circuit 320 overbus 317 as value PD. The PMT address number associated with the peak PMTis output by circuit 320 over bus 312 as value PA. To support analogglobal energy data being transferred over a digitizer channel of 280(i),the peak detect 320 can be disabled for a given PMT address. Bus 347 isalso coupled to a global energy accumulation (GE accumulation) circuit330. Circuit 330 sums the corrected integrated channel signals outputover bus 347 for each of the PMT channels for a given event. The outputof circuit 330 is the digitized global energy GE (which is a digital sumof all of the PMT's digital integrated signals) and is transferred overbus 322 which forms output GE and also is coupled to the dynamiccompression table 355.

Buffer 325 stores the digital integrated signal value of each PMTchannel correlated with the appropriate PMT address received over bus347. Buffer 325 can be implemented in RAM or other memory storagedevice. The PMT integrated signal values for all channels are stored inbuffer circuit 325. The address of the peak PMT is output over bus 312to circuit 335 which is the PMT address table. Circuit 335 contains alookup table that outputs a PMT cluster based on a peak PMT addressinput from bus 312. The PMT cluster is a collection of PMTs whoseintegrated channel responses (as well as a total energy for the event)are used to perform the DEP computation to determine the spatiallocation of the event (e.g., the centroid of the cluster). By using alookup table at circuit 335, the present invention is able to providespatially variable cluster shape and to vary spatially the number ofPMTs that form a given PMT cluster. Within the scope of the presentinvention, for an input peak PMT address, the shape of the resultant PMTcluster and the number of PMTs that make up the selected PMT clustervary based on the spatial location of the peak PMT address within theoverall PMT matrix. It is appreciated that the PMT address table 335 mayvary the PMT cluster associated with the peak PMT address based on aselection for high or low resolution wherein a low resolution mode mayrequire 7 PMTs per PMT cluster where a higher resolution mode mayrequire 9 to 19 PMTs per PMT cluster. Therefore, a resolution indicationsignal (not shown) is also input to the table 335. It is appreciatedthat in lieu of a resolution indicator signal, the entire table 335 maybe reloaded with data for different desired resolutions. In such casethe resolution signal is not used as an addressing signal but onlyinitiates the downloading of the new information.

A sequence counter 390 is coupled to the PMT address table 335 via bus367. The PMT address table 335 controls (1) the number of PMTs in theselected PMT cluster for the given event and (2) the type value of thePMT cluster (which is then stored in circuit 340 and held throughout thespatial computation for a given event). The PMT address table 335 alsocontains the address of the analog global energy channel. The sequencecounter 390 then counts, sequentially, from one to the number of PMTsthat are associated with the selected PMT cluster and sequentiallypresents each count value over bus 367. In one embodiment, the PMTaddress table 335 is itself addressed by two values, (1) the MSB of theaddress that originates from the peak PMT address value over bus 312 and(2) the LSB of the address value that originates from the count value ofthe sequence counter 390 over bus 367. The last entry within the PMTaddress table for a given peak PMT includes a stop code that indicatesthe end of the PMT cluster constitution for that peak PMT address. Thecentroid computation circuitry therefore stops (e.g., is terminated)when the stop code is reached (or equivalently when the maximum countvalue is reached as reported by the table 335).

The PMT address table 335 outputs over bus 372, in sequence based on thesequence counter 390, the PMT addresses of each PMT of the PMT clusterused in the spatial computation for a given event. The order in whichthese PMTs are presented over bus 372 is governed by the lookup tablestored in the PMT address table 335 based on the peak PMT address valuefrom bus 312 and the count value over bus 367. The PMT address valuesoutput from circuit 335 are also coupled to data buffer 325 to addressthis memory circuit which will output the appropriate integrated channelsignal value over bus 352 for that PMT. This output is used in the DEP'sspatial computation.

Referring still to FIG. 2D, the PMT cluster type value is output fromthe PMT address table 335 to the memory circuit 340 which holds the PMTcluster type value throughout a centroid (e.g., coordinate) computation.The PMT address values over bus 372 are also fed to a weight tablecircuit 345 that contains a lookup table correlating PMT address valuewith a given x and y weight value used for the spatial computationcircuitry of the present invention. By providing the lookup table withincircuit 345, the weight associated with a given PMT depends on itsaddress value and is correlated with the spatial location of the PMT andit is also dependent on the PMT cluster type value from bus 377.

The weight value associated with a given PMT address can also vary basedon the type value stored in 340 which is based on the peak PMT for thePMT cluster of the given event; this is accomplished by the typeregistration circuit 340 of the present invention. Therefore, if thepeak PMT address corresponds to be an edge or corner PMT, then the PMTcluster will be of a special type and the weights associated with thePMT addresses of the resultant PMT cluster can be adjusted to accountfor the missing PMTs of the centroid computation (e.g., the PMTs thatare not available due to the edge or corner location of the peak PMT).Therefore, the PMT weight value output from circuit 345 depends on (1)the PMT address value and (2) the type value from circuit 340 that isbased on the address value of the peak PMT for the PMT cluster. The PMTaddress table 335 defines the PMT addresses that constitute the selectedPMT cluster.

In one embodiment, the type registration circuit 340 may contain alookup table based on the peak PMT address value output from circuit335. As discussed, the PMT cluster type value is an offset into theweight table 345 for a given PMT cluster and is used to providevariations in the weights output of the weight table 345 for a given PMTaddress value of a given PMT cluster. Circuit 340 is coupled to supplythe offset value to circuit 345 via bus 377. According to the operationof the present invention, for a given PMT cluster, the PMT addressesthat make up the PMT cluster are sequentially output over bus 372 to theweight table and a constant type value is generated and output over bus377 (based on the address of the peak PMT). The weight table 345 thenoutputs an x weight over bus 362 and a y weight over bus 357, for eachPMT of the given PMT cluster.

The x weight value over bus 362 of FIG. 2D supplies a multiplieraccumulator circuit (MACx) 370 for the x coordinate computation and they weight value over bus 357 supplies a multiplier accumulator circuit(MACy) 365. These circuits 370 and 365 are reset and initialized at thestart of the spatial computation for each gamma event. The x and yweight values are utilized in the spatial computations for a given event(e.g., gamma interaction) and specify the amount of contribution a givenPMT's integrated channel signal value should carry (for a given PMT ofthe PMT cluster) in the coordinate computation process.

As discussed above, the PMT addresses of the PMTs of a given PMT clusterare placed over bus 372 in sequence. Bus 372 of FIG. 2D is coupled toaddress the buffer 325 with the PMT address value for each PMT involvedin the PMT cluster. In response to the address value of a given PMT ofthe PMT cluster, buffer 325 outputs the stored and corrected digitalsignal value associated with the PMT (for a given event) as receivedover bus 347. This corrected signal value is transferred over bus 352 toblock 350, to block 355 and to subtractor 391. A circuit 360 is coupledto receive the digital signal values of the PMT's involved in the givenPMT cluster and accumulates these values to provide a local energy (LE)value which is generated over bus 337.

Block 350 is a buffer for containing either a digital or analog globalenergy value when operating in a mode wherein the global energy data istransferred over a digitizer channel and stored in the storage buffer325. In this mode the global energy stored in block 350 is a digitizedversion of the analog global energy signal. Output from the analogsignal A55 (of FIG. 2B) is fed to a preamplification digitizer channelof 280(i) and then over bus 307 to the calibration table 315 and storedin buffer 325. The global energy may also be computed by accumulator 330by adding the values of the integrated signals from each PMT channel.The global energy value is then stored in buffer 350. Therefore, theglobal energy value GE over line 322 can be (1) a digitized value of theanalog global energy value or (2) a digital summed value of the digitalsignals of each PMT channel.

The dynamic compression table 355 of FIG. 2D receives the global energyvalue of the detected gamma event over bus 322 and also receives thedigitized integrated channel signal value for a given PMT of the PMTcluster over bus 352. The dynamic compression table 355 contains alookup table of compensation values for the digitized integrated channelsignals. The output of the lookup table is driven on bus 392 to asubtractor 391 which also receives the signal value over bus 352. Thetable 355 receives as an address the MSBs of the global energy (from bus322) and predetermined bits of the signal value for each PMT over bus352. Via the connection with the subtractor 391, the signal data overline 352 is shifted left by four bits (e.g., multiplied by 16). Theoutput from table 355, in one embodiment, is subtracted from this leftshifted signal value. The output of the subtractor circuit 391 is thedynamic compressed integrated signal value for a given PMT channel andis then fed to the MACx 370 circuit, the MACy circuit 365 and also to anenergy multiplier accumulator (MACz) circuit 360.

The dynamic compression table 355 of FIG. 2D is utilized by the presentinvention to alter the integrated channel signal output from a PMTchannel so that when summed with altered signals from other channels,the summation signal will be more linear in nature. The informationstored in the dynamic compression table 355 that is used to perform thesignal conversion is readily programmable within the present inventionand different conversion data sets may be stored (down loaded) in thetable 355 at one time. Since the conversion data may be altered readily(e.g., a new set can readily be downloaded, if needed) the dynamiccompression table 355 of the present invention is modifiable. Thesubtraction logic 391 is a part of the compression procedure used by thepresent invention and is used in order to reduce the memory sizerequirement of the compression table 355. Therefore, it is appreciatedthat, given a larger memory size, the subtractor 391 may be eliminatedfrom the present invention and integrated into the memory 355 byaltering the data stored therein. The output of the dynamic compressiontable 355 is called the "dynamic compressed" or "compressed" integratedsignal data for a particular channel and is supplied to the centroidcomputation logic via bus 387.

Therefore, as the sequence counter 390 counts through the PMTs of thePMT cluster, the buffer 325 supplies the integrated signal associatedwith each PMT. Also, the weight table 345 supplies the x and y weightvalues associated with each sequenced PMT. The MACx circuit 370multiplies the x weight value and the dynamic compressed signal for eachPMT and accumulates these values for each PMT of the PMT cluster as thesequencer counts. The MACy circuit 365 multiplies the y weight value andthe dynamic compressed signal for each PMT and accumulates these valuesfor each PMT of the PMT cluster as the sequencer counts. The MACzcircuit has two inputs, one is coupled to bus 342 which is programmed toa value of "1," and the other input is coupled to bus 387 and thereforewill accumulate the integrated signal of each of the PMTs of the PMTcluster to generate a value of the local energy (LE) over bus 337.

After the sequencer 390 reaches the last PMT of the PMT cluster, theMACz circuit will output the complete LE value over bus 337 which iscoupled to a 1/LE circuit 385. This is a lookup table that provides theinverse (e.g., (LE)⁻¹) of the LE value. Circuit 385 can also be realizedusing a divider circuit. The value of (1/LE) is then output over bus 382to x multiplier circuit (MULx) 375 and also y multiplier circuit (MULy)380. Circuit 375 multiplies the accumulated result of the MACx circuit370 with the (1/LE) value to generate the normalized x coordinate of thegamma event over bus 327. Circuit 380 multiplies the accumulated resultof the MACy circuit 365 with the (1/LE) value to generate the normalizedy coordinate of the gamma event over bus 332. Therefore, the DEP 300 ofthe present invention computes the spatial coordinates (x, y) and thetotal energy (GE) of each gamma event. Also produced for each event isthe peak PMT energy (PD) which is the peak signal of the integratedchannel signals received by the DEP, peak PMT address (PA) and the localenergy (LE).

The spatial coordinates (x, y) of a gamma event (interaction) arecomputed by the DEP 300 circuit using the below centroid computation:##EQU1## Where: Wx_(i) =the x weight from circuit 345 for the i^(th) PMTof the Cluster

Wy_(i) =the y weight from circuit 345 for the i^(th) PMT of the Cluster

Wx_(n) =the x weight from circuit 345 for the last PMT of the Cluster

Wy_(n) =the y weight from circuit 345 for the last PMT of the Cluster

E_(i) =the integrated signal for the i^(th) PMT of the PMT Cluster

E_(n) =the integrated signal for the last PMT of the PMT Cluster

The DEP 300 operates as discussed above for each detected gamma event ofan imaging session and stores the above information to a computer memorystorage unit. This information is then transferred to correctionelectronics (or CPU system) where the data is corrected for energy,linearity, and uniformity. It is appreciated that a number of well knownmethods and circuitry components may be used for collecting the countdata output from the DEP 300 circuit and for forming an image basedthereon by connecting the data supplied from DEP 300 (e.g., fornonunifomities, etc.) and spatially recording the counts. Any of thesewell known methods may be used in conjunction within the presentinvention.

FIG. 3 illustrates the applicability of the above spatial computationsand gives an exemplary situation. FIG. 3 illustrates a selected PMTcluster configuration as is generated based on the peak PMT address(here it is PMT0) and based on the PMT address table circuit 335. Lowresolution computation is selected in this example so the PMT cluster iscomposed of seven PMTs (six surrounding and one center PMT). The addresstable circuit 335 would also output a type registration to circuit 340to indicate that the PMT cluster is of a symmetrical or normal typebecause the peak PMT is not located on the edge nor in a corner of thePMT array of the detector head 80. An exemplary x axis 410 and y axis415 are shown and the weight values for each of the PMTs for the x and yaxis are plotted along the axis for each PMT.

The exemplary event occurs at point 50 within FIG. 3 and the arrowsextending outward represent the amount of light received by each PMTtoward which the associated arrow points. The weight values for each ofthe PMTs in the x and y directions are also dependent on the spatiallocation of the peak PMT because the location of the peak PMT will alterthe PMT type value which is used (in conjunction with the PMT address)to address the weight table circuit 345.

FIG. 4A and FIG. 4B are flow diagrams illustrating the generalprocessing flow 460 of the present invention. Refer to FIG. 4A where theprocedure enters and receives a signal, over each PMT channel, and afterconverting the signal from current to voltage, performs a computercontrolled baseline voltage offset for each channel at 462 and alsoperforms a computer controlled coarse gain adjustment for each channelat 464. At block 468, each channel is digitized and also triggerdetection is performed at 466 by summing the analog signals just priorto digitization. Dual integration takes place at blocks 470 and 472wherein a first and second trigger may be used to integrate separateevents occurring close in time. Both integration steps output integratedsignals to the two stage FIFO circuit and at 474 these data values, perchannel, are transferred to the DEP sequentially at the completion of anintegration period. The raw data is sampled and made accessible to adata processor at 476 and this data is supplied to the calibration tableat 478 which removes the baseline offset and also performs fine gainadjustment of the digital channel signal.

At 480, the data from the calibration table is stored for each channelin a buffer. At 482, the present invention determines the peak PMT byexamining the channel data from the calibration table and also at 484the global energy is determined by summing the digital data of eachchannel for the event. The peak PMT address is used as a measure of thecoarse spatial location of the gamma event. At 486, the presentinvention PMT address table outputs a PMT cluster type and alsodetermines the constitution of the PMT cluster based on the peak PMTaddress (and the selected resolution, e.g., fine or coarse, in oneembodiment).

Referring to FIG. 4B, the flow 460 of the present invention continues at488 where the circuitry used to perform the centroid computation isreset to initialize for the new computation. At 490, the sequencecounter addresses the PMT address table so that the first PMT address ofthe selected PMT cluster is output. From this value, and also based onthe PMT cluster type, the present invention generates an x weight (Wx)and a y weight (Wy) for the selected PMT address. Also, the buffer 325contains and supplies the stored integrated signal data for this channelat 494 and at 493 the dynamic compression circuit outputs thedynamically compressed signal value for this channel. At 495, the x andy multiplication accumulation circuits are used to multiply the weightvalue times the dynamic compressed signal value and accumulate theresult for the PMT cluster. At 495, a local energy accumulator alsoaccumulates the local energy of the PMT cluster. At 496, the sequencecounter increments and addresses the PMT address table for a next PMTaddress until the PMT cluster is complete (e.g., the stop indicator oftable 335 is reached); flow returns to 490 if the PMT cluster is notcomplete. The above processing then continues with the next PMT addressof the selected PMT cluster.

If the PMT cluster is complete, then flow continues to 497 where the xand y multiplication accumulation circuits are effectively divided bythe local energy to produce a normalized (x, y) spatial coordinate forthe gamma event. At 498, the pertinent information output from the DEP300 (including the (x, y) coordinate and the total energy) is output toa computer system data processor or a correction board that performsenergy, linearity and uniformity correction in known manners.

DATA PROCESSOR

Refer to FIG. 5 which illustrates components of a general purposecomputer system 1112 that is capable of executing procedures of thepresent invention for controlling the DEP 300 circuit (e.g., control ofbaseline offset, gain, and trigger threshold) and for performing otherrecited functions. The computer system 1112 comprises an address/databus 1100 for communicating information within the system, a centralprocessor 1101 coupled with the bus 1100 for executing instructions andprocessing information, a random access memory 1102 coupled with the bus1100 for storing information and instructions for the central processor1101, a read only memory 1103 coupled with the bus 1100 for storingstatic information and instructions for the processor 1101, a datastorage device 1104 such as a magnetic or optical disk and disk drivecoupled with the bus 1100 for storing image information andinstructions, a display device 1105 coupled to the bus 1100 fordisplaying information to the computer user, an alphanumeric inputdevice 1106 including alphanumeric and function keys coupled to the bus1100 for communicating information and command selections to the centralprocessor 1101, a cursor control device 1107 coupled to the bus forcommunicating user input information and command selections to thecentral processor 1101, and a signal generating device ("communicationdevice") 1108 coupled to the bus 1100 for communicating commandselections to the processor 1101. A hardcopy device 1109 (e.g., printer)may also be coupled to bus 1100.

The signal generation device 1108 includes a high speed communicationport for communicating with the DEP 300. Input bus 1120 receives thedata output from the DEP 300 such as signals PA 312, PD 317, GE 322, X327, Y 332 and LE 337. Bus 1120 also supplies the raw data output fromcircuit 310 over bus 397 to the processor 1112. Output from theprocessor 1112 are the control signals for controlling the fine andcoarse baseline offset voltages (e.g., signals 212 and 214) and the PMTgain signal adjustment 220) for each channel. Processor 1112 alsocontrols the trigger threshold 108. The raw data sampled over bus 397and the control signals generated by processor 1112 may be determinedand adjusted in real-time by the present invention.

The display device 1105 of FIG. 5 utilized with the computer system 1112of the present invention may be a liquid crystal device, cathode raytube, or other display device suitable for creating graphic images andalphanumeric characters recognizable to the user. The cursor controldevice 1107 allows the computer user to dynamically signal the twodimensional movement of a visible symbol (pointer) on a display screenof the display device 1105. Many implementations of the cursor controldevice are known in the art including a trackball, finger pad, mouse,joystick or special keys on the alphanumeric input device 1105 capableof signaling movement of a given direction or manner of displacement.The keyboard 1106, the cursor control device 1107, the display 1105 andhardcopy device 1109 comprise the user interface block 93.

DUAL INTEGRATION PER CHANNEL

The present invention provides multiple independent integrators perchannel (e.g., two per PMT) in order to accurately process high countrates and to effectively reduce problems associated with pulse pile-up.Although described in a specific embodiment utilizing two integratorsper channel, it is appreciated that the present invention system may beexpanded to encompass a multiple number of integrators per channel(e.g., three, four, five, etc.). As will be discussed in further detailto follow, for each integrator added an additional stage within theserial latch circuit is required.

FIG. 6A illustrates light intensity response (curve 440) over time fortwo events. The light intensity response is a well known decayingexponential with a time constant, T. Event1 occurs and decays over timeas shown by curve 440. Due to the characteristics of the crystal 81 usedin the detector 80, at the end of 5 T time intervals, most of the usablelight intensity is radiated. However, there is some time value R, lessthan 5 T, that can be used and gives a sufficient amount of energy toregister event1 (e.g., energy associated with region 430). Prior artsystems will attempt to utilize this decreased amount of energy (region430) for registering events during periods of high count rate (e.g., ifa second event occurs before 5 T as shown as curve 435). However, thisis not advantageous because as the separation between events decreasesbelow 5 T, less and less event energy is captured and spatialcomputations become less accurate. Further, at some point, (e.g., lessthen R) the temporal separation between two events will become too smalland neither event can be registered. At higher count rates the spatialaccuracy of the prior art systems decreases significantly.

The present invention, on the other hand, provides a mechanism forintegrating the energy over 5 T for both event1 and event2 because dualintegrators are supplied per channel for all the PMTs of the array.Therefore, one integrator may sample and integrate the light intensityfor event1 over response 440 for 5 T duration and the other can sampleand integrate the light intensity for events over response 435. Duringperiods of high count rate, the present invention does not sacrificeenergy intensity when sampling each event when integrating over twoevents that closely occur in time. Furthermore, since two separateintegrators are used, the present invention can accurately register twoevents even though their temporal separation is less than R. The presentinvention therefore provides higher accuracy at higher count rates overthe prior art.

The present invention dual integrator embodiment utilizing twoindependent integrators is now discussed. As discussed with reference toFIG. 2C, the integration circuitry of circuit 280(i) for a given channelincludes two integrators 238 and 240 each having independentaccumulators and each coupled (via a mux 241) to a two stage sample andhold circuit (latch 242 and latch 244). Each integrator is independentlytriggerable and separately and independently integrates its associatedchannel signal. Trigger signals are transmitted over bus 130 and whenreceived, act to reset the accumulator of an idle integrator. At the endof sampling, for an event, the present invention transfers the data ofthe second stage (244) to the DEP 300, moves the data of the first stage242 to the second stage and moves the data of the accumulator of thecompleted integrator into the first stage of the hold circuit. This way,both integrators may be sampling, simultaneously, different events. Theycan each be independently triggered and at the end of the sample period,the accumulator stores its result in the two stage hold circuit.

The above procedure operates most accurately when the two integratedevents are sufficiently separate from one another such that their energydissipation across the detector does not overlap or interfere in theintegration computation. For instance, refer to FIG. 7 which illustratesan exemplary PMT array. A first event occurs over PMT 7, therefore thePMT cluster 75 is composed of PMTs 1, 8, 20, 19, 36, 18, and 7. Thetrigger pulse resets integrator 238 (per channel) which then integratesthe energy for the first event for all 55 channels. Before theintegration is complete for this first event, a second event occurs overPMT 38, so the PMT cluster 73 is composed of PMTs 38, 37, 22, 23, and39. Integrator 240 (per channel) is reset and integrates the energyassociated with the second event for all 55 channels. Since PMT cluster75 and 73 are sufficiently separate, the contribution of energyassociated with the second event over channels 1, 8, 20, 19, 36, 18 and7 is relatively small and does not interfere with the integrationcomputation for the first event. Likewise, the contribution of energyassociated with the first event over channels 38, 37, 22, 23, and 39 isrelatively small and does not interfere with the integration computationfor the second event.

At the end of the computation for the first event, the integratedchannel signal data of latch 244 is output to the DEP 300, theintegrated channel signal of latch 242 is output to latch 244 and thevalue of the accumulator of integrator 238 is output to latch 242. Atthe end of the computation for the second event, the integrated channelsignal of latch 244 is output to the DEP, the value of latch 242 isoutput to latch 244 and the value of the accumulator of integrator 240is output to latch 242. The first and second events will be processed bythe DEP 300.

FIG. 6B illustrates in more detail the process performed by the presentinvention to perform dual integration per channel. The process shown 501represents the process for integrator A but it is appreciated that theprocess for B (block 516) is identical except that the "B" controlsignals are used. As shown, the process starts at 510 in response to atrigger signal, Start(t0). Then, the A accumulator is cleared (e.g.,integrator 238) by assertion of a CLRACCA control signal and at 512 thisaccumulator is enabled to integrate by assertion of an ENACCA controlsignal. At block 514, if Start(t1) is detected then at block 516 theprocess for integrator B is started and operates concurrently withprocess 501. If not, then at block 518 it is checked if the integrationfor A is complete. If so, not then the process returns to 512 whereintegration continues.

When integration for A is complete, at block 520 a control signal forMUX 241 selects the data from integrator 238. At block 522 FIFO1 (242)is checked if empty and if empty, at block 524 the data is latched intoFIFO1. At block 526 if FIFO2 (244) is empty then data from FIFO1 islatched into FIFO2 at block 528. At block 530 the data transfer fromFIFO2 to the Digital Event Processor is started for all channels. Duringthe period from block 510 to block 522 the A integrator is busy. It isappreciated that there is only one trigger signal and it is classifiedas Start(t0) or Start(t1) by its temporal relationship to the othertrigger signal. During the A busy period, a trigger signal will causethe B process to start.

VARIABLE PMT CLUSTER CONSTITUTION

As discussed above, the PMT address table circuit 335 of the presentinvention contains a lookup table that provides the addresses of thegroup of PMTs that constitute the PMT cluster for a given event based onthe peak PMT for that event (supplied from circuit 320) and based on acount value supplied over bus 357 from the sequence counter 390. Acentroid is computed (using a centroiding computation) based on the PMTcluster to arrive at a spatial coordinate value of the event. In thisway, the PMT cluster constitution of the present invention varies foreach peak PMT. According to the present invention, also associated witheach PMT of the detector array is a type registration or classificationthat describes the type of PMT cluster that is formed.

For instance, the present invention provides four different types ofexemplary PMT clusters: 1) normal; 2) long edge; 3) short edge; and 4)corner. The normal PMT cluster anticipates the peak PMT to be located inan area of the PMT array that can be surrounded by other PMTs such thatthe PMT cluster is substantially symmetrical about both axis. Such a PMTcluster is shown in FIG. 7 as PMT cluster 75 with PMT 7 as the peak PMTwhich is surrounded by outer PMTs. The determination of the peak PMTgives a coarse spatial location for the event and the present inventionPMT address table 335 is able to utilize this coarse location of theevent in determining the PMT cluster constitution for that event so thatthe fine spatial location may be provided via the centroid computationof the DEP 300.

Two PMT cluster types (long edge and short edge) correspond to PMTclusters having peak PMTs located on the edge of the detector PMT array.Edge PMT clusters are not symmetrical about one axis. The PMT array ofthe present invention is rectangular and contains a long edge and ashort edge and depending on the edge location of the peak PMT, the PMTcluster defined thereby can be a long edge PMT cluster or a short edgePMT cluster. The PMT clusters of these types are different because thelight distribution at the long and short edges are different. An edgetype PMT cluster is shown in FIG. 7 as PMT cluster 73 having PMT 38 asthe peak PMT. The fourth type of PMT cluster of the present invention isa corner PMT cluster and the PMT of this type is located in a cornertype of the PMT array, such as PMT cluster 71 of FIG. 7 having peak PMT46. This PMT cluster type is not symmetrical about either axis. It isappreciated that the above PMT cluster types are exemplary and manyother PMT cluster types may be utilized within scope of the presentinvention depending on the particular geometry of the PMT array utilizedand the geometry of the detector head.

It is appreciated that for a given peak PMT address, the type value forthe PMT cluster can change depending on the selected resolution of thespatial computation. For instance, a normal type PMT cluster at highresolution having the same peak PMT may be different from the PMTcluster generated at low resolution for the same peak PMT (which may bean edge type cluster). This is the case because at higher resolution,more PMTs are required (e.g., 17 or 19) to complete the PMT clusterrather than 7 for the low resolution cluster and these additional PMTsmay overrun the edge of PMT array.

Type fields are important because the PMT cluster type will effect the xand y weights assigned to a particular PMT of a particular PMT clusterin the spatial computation. For instance, PMT cluster types that aresymmetrical about only one axis (e.g., edge types) will have modifiedweight values assigned to those PMTs of the PMT cluster that are locatedalong the axis that is not symmetrical. For instance, refer to FIG. 3which illustrates a spatial computation based on a normal PMT clustertype. The spatial coordinate is computed from an average of a sum basedon the weight of a PMT multiplied by the integrated channel signal ofthe PMT for each PMT of the PMT cluster. The weight values for the PMTsmust be adjusted in the computation of a spatial coordinate along anaxis for which a PMT cluster is not symmetrical. Referring to FIG. 3,assume that PMTs 2 and 3 were not available because PMT 0 was locatedalong an edge. The resulting PMT cluster is composed of PMTs 0, 1, 6, 5,and 4 and is not symmetrical about the X axis. For the coordinatecomputation of the X axis coordinate, the average calculation would beskewed or shifted to the left because the PMTs of the right (e.g., PMTs2 and 3) are missing. Therefore, the weight values of the PMTs of theresulting PMT cluster must be reduced to compensate for the skew to theleft. The same is true for corner PMT clusters, however the weightvalues must be adjusted for the computation of both coordinates becausecorner PMT clusters are not symmetrical to the X or Y axis.

The weight adjustment of the PMTs based on the PMT cluster type field ismade by the weight circuit 345 and will be discussed further below.

Refer to FIG. 8 which illustrates the format of the PMT address tablecircuit 335 for low resolution selection. The PMT address table circuit335 is addressed by the peak PMT address value and also addressed by thecount value (here shown from zero to n). Circuit 335 contains an entryfor each of the 55 PMTs of the detector array. For each peak PMT, thecircuit 335 outputs a PMT cluster type value and the PMT addresses ofthe PMT cluster. The output of the circuit 335 is (1) a PMT cluster typevalue and (2) the PMT addresses (a "PMT list") of the PMTs of the PMTcluster defined by the peak PMT address. Since the PMT clusters areprogrammable and of variable size and PMT number, a "stop" indicator isplaced at the end of the PMT list (or included as part of the last PMTaddress entry). Exemplary data is shown in FIG. 8 and corresponds to thePMT clusters shown in FIG. 7. The first entry shown of FIG. 8 relates toPMT cluster 74 of FIG. 7 and PMT 7 is the peak PMT and the PMT clusteris a normal type having PMTs 1, 8, 20, 19, 36, and 18. Entry 38 of FIG.8 relates to PMT cluster 73 of FIG. 7 and is an edge type PMT cluster.Entry 46 of FIG. 8 relates to PMT cluster 71 of FIG. 7 and is a cornerPMT cluster.

The data stored in the memory circuit 335 that is used to formulate thePMT clusters for each peak PMT can be programmable and may be downloadedfrom the computer system 1112. In such an embodiment, different datasetsmay be loaded into the memory circuit 335 for different configurations.Alternatively, the circuit 335 may be implemented using static ROMmemory.

Based on a clock signal, the sequence counter 390 of the presentinvention will present the count field over bus 367 (one at a time) andthis count value will address the circuit 335 along with the peak PMTaddress to output (1) the PMT cluster type and (2) the PMT address ofthe PMT cluster as shown in FIG. 8.

It is appreciated that depending on the desired resolution of the gammacamera, the PMT address table 335 of the present invention will outputdifferent sized PMT clusters for each PMT cluster type. For instance, ifhigh resolution spatial determination is required, then the PMT clusterswill be increased in size to include 17 to 19 PMTs for a normal PMTcluster (instead of seven for normal cluster types in low resolution).Edge and corner PMT clusters will be increased accordingly in number.Therefore, in an alternative embodiment of the present invention, thePMT address table 335 receives an additional signal indicating low orhigh resolution and this signal will address the table to supply theappropriate centroiding information based on the required resolution.Alternatively, the entire table 335 may be reloaded with a differentdata set to vary the resolution.

Given the design of the PMT address table of FIG. 8, the number of PMTsof a given PMT cluster may easily be increased since the stop indicator,which marks the completion of the PMT cluster, is readily adjusted.Further, as stated above, the PMT cluster type corresponding to aparticular peak PMT address may vary from low to high resolutionsettings.

VARIABLE PMT WEIGHTS PER PMT

According to the present invention, the weight table circuit 345 outputsx and y coordinate weights for each PMT of the PMT cluster based on thePMT address and the PMT cluster type information, both of which aregenerated by the PMT address table 335. FIG. 9 illustrates the weighttable circuit 345 of the present invention. Depending on the type of PMTcluster (e.g., normal type, long edge type, short edge type, cornertype, or other type) that the PMT is contained within, the x and ycoordinate weights output from the circuit 345 for the PMT will vary.For each PMT address (e.g., from PMT0 to PMT54), the present inventionprovides a separate and programmable weight value for each coordinatecomputation (e.g., Wx and Wy) that varies by PMT cluster type. Thevalues Wx are output over bus 362 and the values Wy are output over bus357. As will be discussed below, the determination of the peak PMT givesa coarse spatial location for the event and the present invention weighttable 345 is able to utilize this coarse location of the event indetermining the proper weight values to assign the PMTs of the PMTcluster. The fine spatial location is computed via the centroidcomputation of the DEP. In such manner, the PMT address value signalover bus 372 and the type signal over bus 377 address the circuit 345.

Since some PMT clusters are not symmetrical about a given axis, forinstance the X axis or Y axis for an edge type PMT cluster, the weightvalues associated with these axis are adjusted or varied in order tobalance out the coordinate computation. This is accomplished by thepresent invention in order to compensate for the missing PMTs (of oneaxis) that are not available to provide a symmetric computation. Forcorner PMT clusters, the weight values associated with both axis areadjusted to compensate for the missing PMTs (of both axis) needed toprovide a symmetric computation. Typically the weight values aredecreased for certain PMTs in order to perform the above balancing. Thevalues weight table 345 of the present invention may also be developedempirically based known events for certain locations.

Further, other factors such as the crystal boundaries, opticalinterfaces, and PMT photocathode properties can make the PMTcontribution different depending on the location of the event. Since,the peak PMT address gives some indication of the coarse location of theevent, the weight table 345 can compensate for the above factors byproviding variable weights.

Therefore, the present invention provides the ability to adjust or alterthe weight values for a given PMT depending on the PMT cluster type inwhich the PMT is located. Depending on the location of the peak PMT, theweight values for the PMTs used in the centroid computation may alter.These weight values are also dependent on the peak PMT address since thepeak PMT address defines the PMT cluster type within the presentinvention. The ability to assign different weighting factors based onthe peak PMT location permits higher accuracy in the centroidcomputations and reduces the demands on the correction processing steps.This contributes to allowing the detector 80 to have larger field ofview without increasing the crystal dimensions.

The data stored in the memory circuit 345 of FIG. 9 that is used toprovide the variable weights for each PMT can be programmable and may bedownloaded from the computer system 1112. In such an embodiment,different datasets may be loaded into the memory circuit 345 fordifferent configurations. Alternatively, the circuit 345 may beimplemented using static ROM memory.

In operation, as the counter 390 counts sequentially, the PMT addresstable 335 outputs a sequential listing of PMT addresses within the PMTcluster. The type signal generated by bus 377 addresses the MSBs of theof the memory 345 and the PMT addresses are the LSBs. As each PMT isgenerated over bus 372, the memory circuit 345 generates an X and Yweight value (over buses 357 and 362) associated with the current PMToutput over bus 372. This information is fed to the centroidingcircuitry for computation of the spatial coordinate a the gamma event.

AUTOGAIN CORRECTION

As will be discussed further below, the autogain calibration of thepresent invention contain two phases: (1) an initial calibration; and(2) a routine calibration. The routine calibration can be performed withthe collimator installed or with the collimator removed.

The present invention provides a procedure for automatically calibratingthe gain of each PMT channel without removing the collimator during theroutine gain calibration phase. This is beneficial at least since thegain calibration may be performing during periods of quality controlwithout removal of the collimator while imaging a sheet source (e.g.,Co-57) and thus saving time and effort. Further, since collimatorremoval is a laborious and time consuming procedure, the automatic gaincalibration of the present invention provides an efficient mechanism forgain correction. The computer system 1112 adjusts a preamplificationgain (Gp) for each PMT by using coarse and fine gain values. The coarsegain adjustment is accomplished by generating a digital gain value thatis converted to an analog signal by a DAC and applied at input 220 ofthe preamplification circuit (280(i)) for each channel(i). The fine gainadjustment is implemented in the calibration table 315 as a look-uptable. The computer system 1112 stores the current value of both thecoarse and fine gain (Gp) for each channel. The computer system 1112also performs the automatic gain adjustment procedure as will bediscussed in further detail below.

The effective gain associated with each PMT channel has two separatecomponents. The first component is the physical gain associated with theindividual PMT itself (Gt) and this gain is established by the physicalcharacteristics of the PMT. The second component is the correspondingpreamplifier gain (Gp). As is known, the PMT physical gain, Gt, may varyover time and change in the long term (e.g., over hours or days). Thesevariations in the Gt create the problem that the present inventionsolves by adjusting the Gp gain associated with each PMT channelindividually to compensate for the changes in Gt to obtain a stable(fixed effective gain) for each PMT channel.

The preamplification gain, Gp, contains two components. For eachchannel, the first component of the preamplification gain Gp is the gainapplied to the output channel signal within the preamplification stages,280(i), for each channel (e.g., coarse adjustment); this component isapplied at gain circuit 222 via input 220. A second component (e.g.,fine adjustment) of the preamplification gain Gp is supplied by thecalibration table 315. If the gain of a particular channel does not varyby more than some small percentage (e.g., 5%) then the gain value foundwithin the calibration table for that channel is altered, not the gainapplied at the preamplification stage for that channel.

The effective gain, Gt*Gp, is to remain fixed over time to yield thesame output signal of a particular radionuclide emitting photon withdistinct energy. The fixed gain of each PMT channel will give stable x,y coordinates and total energy values which will increase the timebetween required recalibration of the camera's energy, linearity anduniformity correction factors. Another advantage of the autogaincalibration of the present invention is that it brings back the camerato its state when it was initially calibrated and when uniformitycorrection factors were generated.

In view of this, since the gain Gt may vary due to the physicalcharacteristics of the PMT, the present invention compensates for thisvariation by altering the computer controlled value of the Gp gain thatis applied to data from each channel. In this way, the automatic gaincorrection procedure of the present invention will maintain theresultant gain (e.g., Gt*Gp) at a fixed value for each channel. Theactual value of this fixed amount can be determined and/or measuredduring manufacturing or at the site of installation and operation of thescintillation detector. Since energy, linearity and uniformitycorrections are based on a fixed gain of each of the calibratedphotomultiplier channels of the array, it is advantageous to maintainthis calibration throughout the operational cycles of the gamma camerasystem so that the gains of the PMT channels are closely calibrated tomatch the gain when the energy, linearity and uniformity correctionfactors were calibrated.

Central and Obscured PMTs.

FIG. 10 illustrates an exemplary PMT array of the present invention aswell as a collimator 83 that is positioned in front of the PMT array.The collimator 83 contains an inner region 615 composed of a honeycombof holes with lead septas allowing radiation of a particular incidentangle to pass there through. The collimator 83 also contains a secondregion (edge region) 83 that is solid lead and partially and fullycovers some edge and corner PMTs. PMTs having their surfaces locatedtotally under region 615 are the central PMTs (centrally located) andPMTs having a portion of their surfaces located under region 610 arecalled obscured PMTs. The present invention provides an automaticprocedure and apparatus for calibrating the gain factors of each PMT ofthe array (including those PMTs that are obscured) while leaving thecollimator installed during the routine calibration procedure.

Each PMT has an area 570 associated with it directly over the PMTsurface, see region 570 of PMT 24 for instance(in FIG. 10). Althoughonly one area 570 is shown, it is appreciated that each PMT of FIG. 10has an associated region 570 located above its individual surface. Theactual shape and size of the region 570 can be varied (e.g., it maycircular, square, hexagonal, etc.). In the preferred embodiment of thepresent invention, this area 570 is circular and covers substantiallyall of the area above a given PMT. This central region 570 will be usedfor calibration of the preamplification gains (including adjustment ofthe calibration table) of each PMT when the collimator is removed andeach centrally located PMT when the collimator is installed. (Some PMTscan be located only partially over the NaI crystal layer and thereforewill receive less light.)

Strip Regions 910.

FIG. 11 illustrates specialized regions 910 (one associated with eachobscured PMT) utilized by the present invention for performing gaincalibration for the obscured PMTs of the scintillation detector. FIG. 11illustrates two obscured PMTs 39 and 40 that are obscured by thecollimator's solid lead region 610 (when installed) as well as a groupof centrally located PMTs (0-6) that are situated inside region 615where the collimator consists of holes permitting radiation to passthrough. The region 910, or "strip" area is associated the obscured PMT39 but a separate strip area 910 is also associated with each obscuredPMT of the scintillation detector. FIG. 11 illustrates only one suchstrip region 910 associated with obscured PMT 39. This region 910 may beof a variety of different geometries and in the preferred embodiment issemi-annular and extends in an arc fashion surrounding a region that islocated substantially equidistant from the center of its correspondingobscured PMT 39.

It is appreciated that this strip region 910 may vary in size and shapeconsistent with the present invention. However, in a preferredembodiment of the present invention, this region 910 is located suchthat it extends substantially equidistant from the center of itsassociated PMT and generally adopts a thin shape as shown in FIG. 11. Afurther requirement of strip 910 is that it extend substantially (if notfully) within the region 615 of the collimator that allows gamma rays topass through.

When the collimator is installed onto the scintillation detector, thepresent invention is not capable of performing gain calibration on theobscured PMTs utilizing an area located directly over their surfaces,such as region 570, because no gamma events are detected under region610 due to the presence of the collimator's edge. Therefore, the presentinvention performs gain calibration for obscured PMTs by recordinginformation associated with gamma events that are detected within theexposed strip area 910 associated with each obscured PMT. In general,the present invention measures the integrated channel response of anassociated PMT, e.g., PMT 39, in response to all gamma events that occurwithin the strip area 910 over a sample interval during the initialcalibration. This information will be used to adjust the gain ofobscured PMT 39.

PMTs of a PMT cluster detect different amounts of energy responsive to agamma interaction depending on their spatial relationship to theinteraction. Refer to FIG. 12 which illustrates an exemplary and typicalPMT cluster 553. For a given gamma interaction (e.g., 140 keV),approximately 4000 separate 3 eV visible scintillation photons areemitted from the crystal 81 and of which only 50% are detected.Depending upon the size of the PMTs, if the gamma interaction occurswithin the center of region 570, the center PMT will collect and reportapproximately 45 percent of the detected light energy. The surroundingadjacent six PMTs each collect approximately 8% of the detected energy.For a number of detected gamma events located near the center of region570 of the center PMT, the center PMT should produce an integratedoutput voltage proportional to approximately 1000 3 eV light photons.Each of the adjacently surrounding PMTs should output a integrated meanvoltage proportional to 160 3 eV light photons for the gamma eventsoccurring within 570.

Refer to FIG. 13A, FIG. 13B, and FIG. 13C which illustrate processingsteps executed by the computer system 1112 of the present invention forperforming automatic gain calibration/adjustment of the preamplificationgains Gp (e.g., associated with circuits 280(i) and the adjustmentwithin the calibration table 315) for each channel of the scintillationdetector.

FIG. 13A illustrates the process flow 950 of the present invention thatis used to perform initial or original calibration of the scintillationdetector and is typically performed prior to the energy, linearity, anduniformity calibration. The energy, linearity, and uniformitycalibration can be performed at the manufacturing site or can beperformed at the operational site of the camera system. This is doneprior to operational imaging of a patient. As shown, at step 957, thecollimator is removed (if present) from the scintillation detector andthe array of photomultipliers is irradiated with a uniform flood fieldof known energy gamma radiation (e.g., a known isotope is utilized). Forall events that are detected within the regions 570 of FIG. 12 (for eachPMT), the total energy of the events are recorded, individually, intomemory 1102 and associated with the appropriate PMT such that adistribution is formed for each PMT of only those events that weredetected above the PMT. The memory 1102 stores the information in amatrix form that associates a given PMT with a distribution of the totalenergy of gamma events detected within region 570 for that given PMT. Atblock 957, a measured peak total energy value of the distribution isdetermined by the computer system for each PMT.

Since a known isotope is utilized, its total energy is known (e.g.,nominal total energy peak). At block 959 for each PMT, the measuredtotal energy peak value, as reported by the scintillation detector atblock 957, is compared by the present invention against the known("nominal") total energy peak value associated with the isotope. Withinblock 961 of FIG. 13A, the gain of each PMT channel is then adjustedsuch that the measured total energy peak value matches the known ornominal total energy peak value.

For instance, since the energy detected by the scintillation detectorresponsive to each gamma event of the flood field is known (e.g., X),the peak energy calculated and associated with all PMTs is comparedagainst the known value (X), and the gain (Gp) of the PMT is adjusted upor down accordingly such that the measured value and the nominal valueare equal. It is appreciated that both the gain values associated withthe preamplification circuits 280(i) and also associated with thecalibration table 315 are adjusted at block 961. The gain valuesassociated with the preamplification circuits are a coarse gain and thegain values of the calibration table 315 are fine adjustment values. Atblock 961, the gain values are also applied to the preamplificationchannels and the calibration table 315 for each PMT channel.

Refer to FIG. 12 which illustrates a PMT cluster of PMTs 0, 2, 10, 11,12, 4, and 3 (center). The total energy of events occurring withinregion 570 of PMT 3 is recorded and stored over a number of events and ameasured peak energy value is determined from the energy distribution(at block 957). For events occurring within region 570 for PMT 3, PMT 3is inherently the central PMT (of the PMT cluster). Therefore, it isexpected that the central PMT detects approximately 45% of the lightenergy (X) detected by the detector. If the total energy of the detectedevent (at region 570) is 5% higher than the expected value of X, thenthe gain Gp of the center PMT 3 is adjusted downward by 10% (at block961) because approximately 45% of 10% adjustment will equally compensatefor the detected 5% deviation. This is performed for each of the 55 PMTsof the PMT array.

However, the above gain (Gp) correction ignores the contribution of theperipheral PMTs. It is true that one or more of the peripheral PMTs(e.g., 0, 2, 10, 11, 12, or 4) may have a deviant gain which, forexample, may cause a 5% energy deviation for events detected within 570of PMT 3. If this is the case, then the above correction to the gain(Gp) of PMT 3 may have been made in error. Therefore, the presentinvention, at block 963, performs the above gain calibration/adjustmentfor each of the 55 PMTs a number of different times in an iterativeprocedure (e.g., 10 to 20 times but the number is programmable) toaccount for the possibility of inaccurate adjustments. Based on thisiterative procedure, the effects of inaccurate gain adjustments becomesignificantly reduced as each PMT is eventually adjusted as a center PMTfor each iteration. At the end of the processing of the last iteration,the final preamplification gain (Gp) assigned to each PMT is recorded inmemory 1102 (or other storage unit) at step 965. Each gain value isstored associated with its PMT.

Referring to FIG. 13A, block 967 is then entered. The collimator remainsremoved from the detector and the PMT array is again illuminated withthe same flood field radiation. For each "obscured" PMT (e.g., notobscured during block 960, but obscured when the collimator issubsequently installed), a region 910 is defined that extends into thefield of view of the detector that will encompass the open region 615 ofthe collimator (when installed). For each "obscured" PMT, each gammainteraction that occurs within its region 910 is recorded by thecomputer system 1112 at block 967. The integrated channel output forthat "obscured" PMT is then recorded to memory 1102 and an energydistribution is formed wherein the peak energy value is then determinedat block 969. In the preferred embodiment, the "obscured" PMT is aperipheral PMT by definition for all gamma events that occur within itsassociated region 910, therefore, the "obscured" PMT is expected toreceive a fraction, C, of the total event energy (wherein C is less than10 percent). For the example of FIG. 11, this would represent afraction, C, of the value X (2000 photons of 3 eV each) as reported bythe detector. The measured peak integrated channel signal detected atblock 960 for the "obscured" PMT should register roughly a fraction ofX, or C*X.

For instance, (see FIG. 11) for all events occurring within the region910 associated with PMT 39, the integrated channel signal outputassociated with PMT 39 is recorded until a distribution is formed inmemory 1102. This is performed for each obscured PMT (with respect toits associated strip area 910) over a number of gamma interactions(e.g., 500, but the number is programmable). The peak PMT signal of thedistribution of integrated channel signals for each obscured PMT is thenrecorded into memory 1102 or other storage unit associated with its PMT.Processing of block 950 then returns.

At the completion of the process 950, the present invention calibrateseach PMT tube of the array according to events that occur over each PMTand records this gain value in memory. Process 950 also records the peakintegrated channel signal for each obscured PMT in response to eventsthat are detected with the obscured PMT's individual strip region 910.

Refer to FIG. 13B which illustrates the routine calibration process 970of the automatic gain correction of the present invention when thecollimator is removed. Routine calibration 970 can be performed at theoperational site for calibration of the scintillation detector while thecollimator is removed. The processing of blocks 971-979 are analogous tothe processing tasks 957-965 of FIG. 13A. Therefore, at block 979 ofFIG. 13B, the connected gain values are stored in memory 1102 andimplemented within the preamplification stages 280(i) and thecalibration table 315 for each channel. Since the collimator is removed,there are not any actual obscured PMTs.

Refer to FIG. 13C which illustrates the routine calibration process 980of the automatic gain correction of the present invention when thecollimator is installed. Routine calibration 980 can be performed at theoperational site for calibration of the scintillation detector while thecollimator installed. At block 981, a uniform flood field source isplaced in front of the collimator. Interactions occur over the PMT arrayand the total energy of events detected over the center region (570) ofeach central PMT (e.g., non-obscured PMT) are recorded in memory 1102and associated with that PMT. A distribution is recorded for eachcentral PMT and a measured total energy peak value is determined foreach central PMT. Block 981 is analogous to block 957 except theprocessing for block 981 is applicable only to central PMTs. Thepresence of the collimator does not interfere with this process becausethe centrally located PMTs are under the open section 615 of thecollimator.

At block 983, for each obscured PMT, the present invention records theintegrated channel signals from the obscured PMT only for gamma eventsthat occur within that obscured PMT's associated strip region 910. Aseparate distribution of integrated channel signals is recorded for eachobscured PMT and stored in memory 1102 (or other storage device). Asstated above, the strip regions 910 associated with the obscured PMTsare aligned with the central portion of the collimator. The processingof block 983 is analogous to block 967, except at block 983 thecollimator remains installed. It is appreciated that blocks 981 and 983,of process 980, can occur coincidentally.

At block 985 of FIG. 13C, the measured total energy peak values of thecentral (e.g., non-obscured) PMTs as reported from block 981 arecompared against the known (nominal) total energy peak values for eachcentral PMT channel. The gain value for each central PMT is thenadjusted accordingly. The processing of block 985 is analogous to theprocessing of block 959 except only central PMTs are processed in block985. The new computed gain values for the central PMTs are stored inmemory 1102 (or similar storage device).

At block 987 the present invention, for each obscured PMT channel,compares the measured spectrum peak value (that was measured withinblock 983) against the stored peak value associated with that obscuredPMT channel (that was stored by block 969). At block 987, the presentinvention computes a new gain (by increasing or decreasing the currentgain) for each obscured PMT such that the measured value matches thestored value for each obscured PMT. This can be performed by a straightratio computation (e.g., if the measured spectrum peak value reportedfrom block 983 is 10% larger than the stored value from block 969, thenthe current gain for that obscured PMT is reduced by 10%, etc.). Atblock 987, the newly computed gain for each obscured PMT is thenrecorded in memory 1102 (or similar storage device).

At block 989 of FIG. 13C, the present invention then applies the newlycomputed gains of: (1) the central PMTs as computed in block 985; and(2) the obscured PMTs as computed in block 987, to the appropriatepreamplification circuits 280(i) and the appropriate locations withinthe calibration table 315 for each channel. The preamplification circuitcontains the coarse gain control while the calibration table 315 providefine gain control. At block 991, the present invention performs theprocessing in an iterative manner over a number of times (a programmablenumber) to refine the calibration and adjustment. When done, processingflows to block 993 where the newly adjusted gain values for each channel(central and obscured) are stored in memory 1102 (or other storageunit).

It is appreciated that processes 970 and 980 can be performed by thecamera system at any time. By performing the process 980 before imagingsessions of the gamma camera system, the preamplification gains of thePMT array are calibrated automatically such that the responses of thePMTs remain essentially the same as when they were calibrated at thefactory. This will cause the scintillation detector to be calibrated tomatch the response it had when the energy, linearity, and uniformitycorrection values were created. This match between the energy,linearity, and uniformity correction factors and the detector responseimproves image quality.

In an alternative embodiment of the present invention, in lieu of usinga peak energy determination of the distributions of processes 959, 969,973, 985, and 987, an average or mean computation can be utilized. A"representative" data value of an energy or signal distribution cantherefore be the peak, average, mean, or similar function, of suchdistribution.

In another alternative embodiment of the present invention, theintegrated channel signals for each obscured PMT are not measured toperform the gain connection for these PMTs. In lieu of the integratedchannel signals being recorded, the total energy of the events occurringwithin the strip regions 910 are recorded and an representative energyvalue is computed for this distribution for each obscured PMT. The aboveis performed for both blocks 967 and 983 such that the total energy ofeach gamma event is recorded and compared.

It is appreciated that the data related to the integrated channelsignals that is needed by the above procedure is gained via the computersystem 1112 from the DEP 300 from bus 397 (see FIG. 2D). The totalenergy for a given event is supplied over bus 322 of DEP 300. In eachcase where channel information is gathered for gamma events detectedwithin specific XY regions, the present invention provides a coincidencecircuit for comparing the XY coordinate of the event as output from bus327 and 332 and the circuit compares this coordinate value against theknown region (e.g., area 570 or strip region 910). If a match occurs,then the desired data is sampled either via bus 322 for the total energysignal or via bus 397 for the specific channel data. Alternatively, thisspecific channel data can also be supplied directly from buffer 325 ofthe DEP 300.

VARIABLE DYNAMIC COMPRESSION

The present invention DEP 300 circuit provides a programmable, variabledynamic compression table 355 (FIG. 2D) so that the energy response of aPMT can be adjusted using a programmable digital function stored inmemory rather than using dedicated analog hardware, as in the prior art.In effect, the dynamic compression table 355 of the present invention isutilized in place of the well known analog breakpoint driver circuits ofthe prior art. The breakpoint driver circuits of the prior art arecomposed of analog circuits (including diode networks) for adjusting thePMT output. However, opposite to the present invention, the prior artcircuits are not programmable or readily modifiable.

The dynamic compression table 355 may be composed of programmable memoryor static memory (ROM or PROM). If programmable memory is utilized, itmay be composed of a number of well known memory types, such as RAM orEEPROM.

The dynamic compression table 355 of the present invention performs twotasks: (1) it reduces or eliminates noisy signals from PMTs far from thepoint of the gamma interaction; and (2) its shapes the remaining signalsin order to obtain a more uniform spatial response across the detector.Digital implementation of this procedure allows the use of relativelysimple linear preamplifiers thus reducing the complexity and cost of theanalog electronics, and improving the long term stability of the system.The Dynamic compression table 355 is utilized to alter the signal outputfrom each PMT channel to remove nonlinearity so that when the signalsummed with other altered signals, the resultant summation signal ismore linear. Refer to FIG. 14A which illustrates the normalized energyresponse of a PMT based on the distance from the PMT center at which agamma interaction occurs. The response 652 is the signal beforecorrection by the dynamic compression table and response 650 illustratesthe corrected or compressed response as output from the compressionlogic of the DEP 300. Due to the characteristics and physics of the PMT,the response is overly elevated for events that occur near the center ofthe PMT, therefore the compression logic reduces this responseaccordingly.

Refer to FIG. 14B. The overly elevated response of the PMT tube near thetube's center poses a problem because when summed with other neighboringPMT's responses, the summation signal is not linear. The accuracy of thecentroid computation depends, in part, on the linearity of the summationsignal as evidenced from the centroid computations as previouslydescribed herein. FIG. 14B is a plot of PMT response over PMT positionfor 5 PMTs in a particular single axis. Response 654 is the summation ofeach of the 5 PMT signals. FIG. 14B illustrates that the summationsignal 654 (sum of the PMT responses 655-658) can be slightly nonlinear,and this nonlinearity tends to reduce the spatial accuracy of thecentroid computation. The dynamic compression table 355 and subtractorcircuit 391 provides an adjustment to the input PMT integrated channelsignal over bus 352 to allow a more linear summation signal. In affect,after compensation by the dynamic compression table 355, the summationsignal 654 has a more linear slope.

FIG. 15A illustrates a graph of the input energy response (normalized toglobal energy) 664 of a PMT channel and the desired normalized alteredresponse 662 as a result of the dynamic compression procedure of thepresent invention. As shown, for moderate energy levels the higher theenergy the more the attenuation. Since the dynamic compression procedureoperates on normalized values, the dynamic compression table 355receives, as an input, the global energy over bus 322. The dynamiccompression table 355 outputs, over bus 392, the value of the offset ordifference between signal 664 and 662 (for a given input signal over bus352) multiplied by the global energy (GE). Therefore, the output of theenergy table 355 of the present invention is not normalized. Thesubtractor 391 then subtracts this output of table 355 from theuncorrected PMT channel signal supplied over bus 352. The result is acorrected or compressed integrated channel signal over bus 397. FIG. 15Billustrates the response of the conversion utilized by the presentinvention for normalized input signals less than 15 percent of the maxsignal value. As shown, the corrected energy curve 662 is slightlyhigher in value than the input or uncorrected curve 664.

In a particular embodiment, the compression table 355 is 6×8 ×14 (32 k)in size and is addressed by the 6 MSBs of the global energy signals(322) and the 8 MSBs of the calibrated channel signal (352). Each 2 bytecell contains a 14 bit offset value which is subtracted from the 10 bitvalue of the channel signal 352. The dynamic compression table 355 iscalculated to 14 bits of precision: 8 significant bits plus 6 fractionalbits. The finer sampling allows the X and Y weighted sums to take on agreater number of values, reducing the quantization effects of theinteger position calculation. The output of the offset value from table355 is 16 bits and the input to the subtractor from bus 352 are the MSB12 bits. The output from the compression table 355 of the presentinvention is at a higher resolution than the signal over line 352.

An exemplary and general compression procedure utilized by an embodimentof the present invention is illustrated as the response shown in FIG.15A and FIG. 15B. Since the dynamic compression is a non-linearfunction, the PMT integrated channel signals are scaled by the globalenergy prior to dynamic compression. The scaling allows the dynamiccompression function to be independent of energy, which is animprovement over the non-linear amplifiers of the prior art.

Within this exemplary procedure, the integrated channel signal suppliedover line 352 (E_(pmt)) is scaled, normalized, by the global energy (GE)relative to the nominal peak global energy (ge) by:

    E.sub.in =E.sub.pmt *(ge/GE)

The value ge is constant and an exemplary value is supplied below. Thescaled signal, E_(in), is passed to the dynamic compression (rolloff)procedure, D, as shown further below.

    E.sub.fn =D(E.sub.in)

Finally, the compressed signal is scaled back, denormalized, to itsoriginal level:

    S=E.sub.fn *(GE/ge)

where S is the signal that is output over line 387 from the subtractorcircuit. The exemplary general roll off procedure of the presentinvention is shown below as:

    D(E.sub.in)=[SHi*e.sup.-(SHi*RHi)/SP ]+[SLo*[e.sup.-(SLo*RLo)/SP -1]]-bais*SP

Where:

SLo=LoThresh*SP-E_(in) (for E_(in) <LoThresh*SP)

SLO=0 (for E_(in) >=LoThresh*SP)

SHi=E_(in) -HiTresh*SP (for E_(in) >HiTresh*SP)

SHi=0 (for E_(in) <=HiTresh*SP)

According to the general procedure, SP is the nominal peak calibratedPMT signal and the each of the other parameters is expressed as afraction of SP. LoThresh is the starting point of the low-end rolloffand HiTresh is the starting point of the high-end rolloff. RLo is thedegrees of low-end rolloff. RHi is the degrees of high-end rolloff. Biasis the DC bias to be subtracted from the rolled off function. Preferredresults have been achieved with the following parameters (however eachis programmable and adjustable within the present invention):

ge=180

SP=200

LoTresh=0.08

HiTresh=0.25

RLo=2.5

RHi=0.35

Bias=0.05

In the computation of the preferred embodiment of the present invention,the compression table 355 contains only the offset value that issubtracted from the channel signal, e.g., the difference between lines662 and 664 of FIG. 15B. Therefore, the actual value stored in thelookup table 355 is computed based on the difference between the aboveprocedure and the channel input signal, times the global energy GE. Thisis shown below:

    Data output over bus 392=[E.sub.in -E.sub.fn ]*[GE/ge]

The output over bus 392 is then subtracted from the uncorrected energyof the particular PMT channel. Therefore, the procedure belowillustrates the corrected or compressed signal:

    Data output over bus 387=E.sub.pmt -Data output over bus 392

The dynamic compression circuit 355 of the present invention ensuresthat the above subtraction does not yield a negative number. Anynegative number output is zeroed.

The above procedure an be reduced and given below assuming a normalizedinput signal from the range 0 to 100:

    E.sub.fn =[E.sub.in *e.sup.-(0.35*TH)/100 +TL*(e.sup.-(2.5*TL)/100 -1)]-0.05

where:

E_(in) =E_(pmt) /GE

TH=E_(in) -25, if E_(in) <25; and TH=0, if E_(in) <=25

TL=8-E_(in), if E_(in) <8; and TL=0, if E_(in) >=8

0.05=small baseline offset

The input to the compression table 355 is GE (over bus 322) and E_(pmt)(over bus 352) for a particular PMT channel. The output of the table 355is shown below. The actual value stored in the lookup table is computedbased on the difference between the above procedure and the normalizedinput, times the global energy GE. This is shown below:

    Data output over bus 392=[E.sub.in -E.sub.fn ]*GE

The output over bus 392 is then subtracted from the uncorrected energyof the particular PMT channel. Therefore, the procedure belowillustrates the corrected or compressed signal:

    Data output over bus 387=E.sub.pmt -Data output over bus 392

The dynamic compression circuit 355 of the present invention ensuresthat the above subtraction does not yield a negative number. Anynegative number output is zeroed.

It is appreciated that given sufficient memory size within circuit 355of the DEP 300, the data can be configured such that the global energyvalue GE is input, along with the channel data of bus 352 and thecompression table 355 can then output the corrected value of the PMTchannel data directly over bus 387. In this embodiment, the subtractor380 is not utilized. Also, the data of the dynamic compression table 355of the present invention can be further optimized by modifying theprocedures illustrated above. These modifications or optimizations mayinvolve empirical data that is particular to a camera system oroperating environment.

As shown in FIG. 15C, another dynamic compression procedure isillustrated that is stored in the dynamic compression table 355 and usedin a similar manner with similar mechanisms as shown and discussedabove.

Given the above procedures, the actual data tables (addressed by GE andE_(pmt)) may readily be determined by one of ordinary skill in the artand are exemplary only. Because the table 355 is programmable, the levelof resolution desired by any one embodiment is variable depending on theavailable size of memory for circuit 355. According to the presentinvention, the data of the dynamic compression table 355 is generated bydata processor 1112 according to a programmable set of parameters andthe compression procedure shown above and this information is thendownloaded into the memory 355. If RAM is utilized, then the actualcorrection data can be stored in a disk storage 1104 or ROM 1103 andthen down loaded into the memory 355 before the gamma camera system isused. Therefore, the data processor 1112 may store a variety ofdifferent procedures and/or correction data tables and a user can selectamong them for generating tables of correction data and for loading theminto memory 355 for use in the DEP 300 as system calibration. In thisalternative, the dynamic compression table 355 is extremely flexible andprogrammable. Alternatively, when the memory 355 is EEPROM, the data isprogrammed into the memory 355 before use and can be reprogrammed by atechnician.

AUTOMATIC BASELINE COMPENSATION

Within the present invention, a slight DC offset (baseline offset) ismaintained within each preamplification stage for each PMT channel via adigital input to a DAC which controls inputs 212 and 214 of FIG. 2C.Shown in FIG. 16 is an exemplary PMT analog channel output voltagesignal 680 as sampled by ADC 236 (of FIG. 2C). Due to the integrationprocesses performed for an event, negative voltages of signal 680 areunwanted, become clipped, and lead to inaccurate integration results.Due to circuit drift of the amplifiers of circuits 280(0) to 280(54)(e.g., due to temperature, current flow, etc.) the baseline of thevoltage signal 680 as sampled by the ADC 236 may vary and, if leftuncorrected, may dip below zero. The baseline shift of an individual PMTis typically a fraction of a channel signal, but the sum of the shiftsover an entire detector can move the global energy peak by severalchannels. Baseline shifts also result in image registration variations.Drift may be compensated for by the use of expensive amplifier circuits,but since a separate set of amplifiers is required for each channel,such a solution is not economically practical. The present inventionprovides an automatic procedure and circuitry for baseline compensationin order to adjust for this variable drift by measuring and adjustingthe baseline voltage in real-time.

In effect, the present invention inserts (via inputs 212 and 214) a baseline compensation voltage amount called Vm in order to adjust the analogsignal per channel. Each channel has an independent Vm. This is shown inFIG. 16. Line 682 shows true zero voltage and the signal 680 is adjustedby an amount Vm. After digitization and integration, this offset valueVm must be subtracted from the total, resulting in the net channelsignal. Baseline subtraction is performed in the calibration lookuptable circuit 315 of the DEP 300 for each channel, and is based on theparticular Vm inserted for that channel.

The amount of voltage offset applied at 212 and 214 provided by thepresent invention varies, as discussed below. However, there is an idealnumber Vm, that is computed based on the level of expected noise withinthe dynamic range of the ADC 236. Since this dynamic range is within1024 units, in one embodiment, and since the expected noise percentageis roughly 8 percent, the digital value of Vm is approximately 78 unitswithin the dynamic range of the ADC 236. Of course this value will varydepending on the specific implementation of hardware utilized. Thisvalue is initially used as an offset to control inputs 212 and 214 (aseparate input is used for each channel) to offset signal 680. The valueof Vm is determined such that signal 680 does not vary below zero. Vm isalso adjusted such that a large portion of the dynamic range of the ADC236 is not consumed by the offset voltage.

The actual value of the voltage offset as measured by the ADC 236 (whenno event is present) may drift. In the present invention, the digitalprocessor 1112 measures the signal output value for a particular channelthat is not substantially receiving energy from a gamma event. Thisvalue should be near the ideal voltage Vm. The sampled voltage iscompared against the ideal voltage and is modified up or down (in areal-time feedback arrangement) to closely match the ideal voltage Vm.In this manner, the present invention compensates for the driftassociated with the electronics of the preamplification stage for eachchannel by modifying the value of the offset voltage. This sample andadjust procedure is performed separately for each channel.

The present invention procedure for monitoring the analog signal of eachchannel and for modifying the baseline of this signal based on the idealbaseline value of Vm is described below. Generally, the presentinvention provides two alternate methods for baseline compensation: (1)a method used during periods of low count rate; and (2) a method usedduring periods of high count rate. The computer system 1112 contains abaseline offset matrix of data values in memory 1102, one for eachchannel, that records the current baseline offset voltage applied atinputs 212 and 214. The values within this baseline offset matrix areupdated by the present invention to compensate for voltage drift of theamplifier stages of the circuits 280(0) to 280(54).

Refer to FIG. 17A which illustrates the channel baseline compensationprocedure 703 utilized by the present invention and performed by digitalprocessor 1112. At 710, the present invention initially determines ifthe count rate detected by the camera system is high or low. High countrate is determined as a count rate that exceeds 75,000 counts persecond, but this count rate is exemplary and is programmable within thepresent invention. Different procedures will be implemented by thepresent invention depending on the camera's count rate. At 715, thedigital processor determines if the count rate is above the threshold(e.g., 75,000 counts/sec) and, if true, block 760 is performed for eventtriggered baseline correction. If the overall count rate detected by thegamma detector is lower than or equal to the threshold amount, then atblock 720, the processor 1112 performs software triggered baselinecorrection. At the completion of a correction procedure, the processingis exited via 799. It is appreciated that the flow 703 of the presentinvention is repeated throughout the operational duty cycle of the gammacamera including periods in which the camera is idle and also duringperiods in which the camera is actively engaged in an imaging session.

Software Triggered.

Refer to FIG. 17B which illustrates the software triggered baselinecorrection procedure that is performed during periods of low count rate.FIG. 17B illustrates the steps performed by the present invention byblock 720. At block 724, for each of the channels of the detector 80,the present invention inserts a software generated trigger that emulatesan actual event trigger that would be detected over line 130 of circuit100 of FIG. 2A. For each channel, the digital processor 1112 in effecttriggers an integrator (e.g., 238 or 240) to begin integrating over apulse period. The integration is performed simultaneously for eachchannel at block 726 (or alternatively in stages) and each channelshould, ideally, integrate over the baseline voltage since statisticallyno event is expected to coincide with the software triggered pulseperiod. The software trigger is also called a false trigger since it isnot initiated based on detection of a true gamma interaction. Since thesoftware triggered integration does not usually coincide with an event,for the most part, the integration performed by the channels will beover a pulse period wherein the baseline voltage should, ideally, bevery close to Vm. This is the case because the channel should not begenerating an output signal due to the expected (e.g., statistically)absence of an event.

At block 728, the data generated from the integration of block 726 isrecovered from the raw FIFO memory 310 (of DEP 300) by computer system1112. This memory contains the integrated result of each channel withoutsubtraction of the Vm offset (which is performed by the calibrationtable 315). Optionally, at block 730, the data for each channel isdivided by the pulse period to normalize the integration in time. Thisstep is optional because if the time period of each integration isknown, then this time period multiplied by the ideal Vm value can beused as a reference by which the base line offset value can becompensated. Either implementation is within the present invention. Atblock 732, the data for each channel is stored into a separate histogramthat is maintained by computer system 1112 in memory 1102 for eachchannel. The histogram records the sampled baseline voltage (for a givenchannel) over the number of samples taken. At block 734, the aboveprocess is repeated for a next sample until a predetermined andprogrammable number of samples is taken. For each sample, the dataassociated with each histograms are increased. An exemplary number ofsamples used by the present invention is 250 samples per channel.Therefore, a particular histogram constructed for each channel by block732 is composed of roughly 250 data points. These histograms aremaintained by the present invention in RAM memory 1102 of the dataprocessor 1112. Assuming 55 channels per detector, there needs to be13,750 false counts per calibration process of the present invention forprocess flow 720.

At block 736 of FIG. 17B, the present invention utilizes the histogramcreated for a particular channel to determine the average value of thesampled baseline offset amount. This determination may be made using anumber of well known averaging, mean or weighted averaging procedures.Any number of different procedures may be utilized consistent within thescope of the present invention at block 736. At block 738, for theparticular channel, the average is compared against Vm (which is storedin memory by processor 1112 and is constant for each channel). If theaverage for the particular channel is greater than Vm, then at block 742the baseline offset of the particular channel which is stored in abaseline offset matrix within memory 1102 (as applied via signals 212and 214) is decreased by one unit and the matrix is updated and block744 is entered. If the average for the particular channel is less thanVm, then at block 740, the baseline offset of the particular channel (asapplied via signals 212 and 214) is increased by one unit and thebaseline offset matrix is updated and block 744 is entered. Otherwise,if the average value of the baseline is equal to Vm, then no baselinecorrection is required and block 744 is entered directly. It isappreciated that there is a separate computer controlled pair of inputs,212 and 214, for each amplification circuit 280(0) to 280(54).

At block 744, the above procedure (to block 736) is repeated for eachchannel, so that the baseline corrections for each channel are adjusted(in a feedback arrangement) separately. At block 746 the procedure isexited and the histograms maintained in memory 1102 for the abovecompensation procedure are reset. Therefore, as can be seen above, thepresent invention measures the channel voltage for each channel duringperiods of no expected events and records and adjusts the baselinevoltage for the channels. Due to drift, these voltages may vary, butshould be constricted to the value Vm to prevent clipping (due tonegative voltage). The present invention provides a measurement andcompensation procedure to effectively maintain the baseline voltages foreach channel to the ideal value, Vm. This is performed using a feedbackarrangement as shown in FIG. 17B thus avoiding the use of expensivecircuitry within the preamplification stages 280(0) to 280(54).

It is appreciated that the present invention, for block 720, does notdetermine in advance if there is a real event occurring in coincidencewith the software trigger generated at block 724. Statistically, it israre (but possible) to have an event occur in coincidence with (andtherefore interfere with) the software triggered integration periodduring periods of low count rate. Should an event occur during block 726of the present invention, that data point will be much larger than theexpected value (due to the PMT responses) and will be effectivelyexcluded statistically from the average (or median) computed for thechannel at block 736. In an alternative embodiment, the presentinvention may perform an additional step of excluding all data pointsfrom the histograms generated in block 732 that exceed a predeterminedthreshold in order to exclude data points corrupted by an event.However, given 250 samples (for instance), the number of data pointscorrupted by an event is very low and is negligible.

Scintillation Triggered.

Refer to FIG. 17C and FIG. 17D which illustrate the procedure 760utilized by the present invention to provide baseline compensationduring periods of high count rates. Procedure 760 utilizes actual eventsto trigger the measurement processes and therefore is called event orscintillation driven baseline correction. The process begins at thedetection of a gamma event at 764 and flows to 766. An event is detectedvia a trigger signal over line 130. For each channel, the trigger pulsecauses each integrator to integrate over the pulse period as describedpreviously herein. Also at block 766, the data from the integration isforwarded to the DEP 300. At block 768, the DEP 300 determines whichchannel was the peak channel (e.g., the peak PMT) for the event by usingcircuit 320 (FIG. 2D) described above.

Based on the peak PMT value (which is reported back to the digitalprocessor 1112), the present invention utilizes a memory circuit 800, asshown in FIG. 18, to determine which PMTs are spatially near to the peakPMT and which PMTs are spatially far from the peak PMT. Far PMTs aredetermined to be those PMTs that receive substantially no (or negligiblylittle) energy from the event and near PMTs are those PMTs that receivesome energy (e.g., less than 0.17%, etc.) from the event. As shown inFIG. 18, there is an entry for each PMT in column 805 and column 810lists those PMTs that are near and column 820 contains a list of thosePMTs that are far. For each peak PMT there is a group of far PMTs thatshould receive substantially no energy from the event. For any givenevent detected at block 764 it is the far PMTs that are sampled by thepresent invention for baseline correction. The set of far PMTs for agiven peak PMT address may be determined experimentally by measuring PMTresponses for a known peak PMT address within a given PMT array.

At block 770 of FIG. 17C, the present invention addresses circuit 800with the peak PMT address (number) to determine the set of far PMTs asreported by column 820. At block 772, for the given event the presentinvention directs computer system 1112 to recover the data associatedwith each of the far channels as stored in the RAW FIFO memory 310 ofDEP 300. These far channels should report essentially an integration ofthe baseline voltage because they received an insubstantial amount ofenergy from the detected event. Optionally, at block 774 the integrationdata of the circuit 310 is divided by the pulse period to normalize thedata in time for each far channel. At block 776, the present inventionadds the value of the sampled voltage to the histogram for each far PMTchannel sampled. At block 778, the above procedure is repeated for thenext sample. This procedure continues until each channel obtains atleast a predetermined and programmable number of data points in itshistogram. An exemplary number of data points is 250 samples perchannel.

It is appreciated that for each gamma event at 764, a different numberand group of far channels will be sampled. Therefore, the number ofhistograms updated for each event will vary. The present inventiontherefore maintains a record of the channel having the least number ofdata points in its histogram and when this channel reaches thepredetermined number (e.g., 250), block 778 will exit to "A" as shown.

Refer to FIG. 17D illustrating the remainder of process 760 of thepresent invention. From "A," the flow continues to block 780 where thepresent invention determines the average (e.g., weighted average,median, mean, etc.) baseline voltage for each histogram for all thechannels. This process is analogous to the process of block 736 of thesoftware triggered procedure 720 except each histogram of block 780 maycontain a different number of data points depending on the spatialdistribution of the detected gamma events. At block 782, the presentinvention determines for a particular channel if its average baselinevoltage amount is larger than Vm. If so, then at block 786 the offsetvoltage stored in the baseline voltage matrix (and as asserted bysignals 212 and 214) is decreased for that channel and block 788 isentered. If the average voltage is less than Vm for a particularchannel, then at block 784 the baseline voltage as stored in thebaseline voltage matrix is increased for that channel and block 788 isentered. If the average baseline offset voltage for the channel is equalto Vm, then no correction is required and block 788 is entered directly.

At block 788, the present invention repeats from block 780 for the nextchannel until all baseline voltages of channels of the detector head arecompensated. At block 790, the process 760 exists. It is appreciatedthat in the event that two gamma events are detected within a smallwindow of time, one event may trigger the calibration sample (e.g.,block 764) and the other event may interfere with the integration of thefar PMTs and therefore increase or interfere with the sample data. Inthese statistically rare cases, as with the software triggeredprocedure, these corrupted data points are effectively excluded (orminimized) statistically via the averaging functions of the presentinvention.

Therefore, as shown above, this embodiment of the present inventionprovides a mechanism and process that maintains the baseline offsetvoltage, per channel, to the ideal value Vm. Drift associated with thetransistors of the preamplification stage of each channel is detectedand corrected by using a feedback arrangement. By using the approach ofthe present invention, the preamplification stages of the detectorcircuitry may be implemented using less expensive (e.g., less drifttolerant) circuitry.

It is appreciated that during periods of high count rate, the softwaretriggered sampling procedure 720 of the present invention becomes lessaccurate as more events occur during the calibration samples. Therefore,the event driven procedure 760 is more accurate for high count rates. Atlow count rates, the event driven calibration procedure 760 is notoperational because the count rate becomes too low, e.g., backgroundradiation on the order of 40 counts/per is too low to perform eventdriven calibration. Procedure 760 requires that each channel be exposedto at least 250 counts per baseline adjustment. Therefore, the use ofboth software driven 720 and event driven 760 baseline compensation ofthe present invention is advantageous for high and low countenvironments.

The preferred embodiment of the present invention, a method andapparatus for improving image quality and resolution within a gammacamera system providing spatially variant PMT cluster constitution andspatially variant centroiding weights for PMTs, is thus described. Whilethe present invention has been described in particular embodiments, itshould be appreciated that the present invention should not be construedas limited by such embodiments, but rather construed according to thebelow claims.

What is claimed is:
 1. In a gamma camera system having a scintillationdetector for receiving gamma radiation, said scintillation detectorhaving an array of photomultipliers wherein individual photomultipliersgenerate channel signals, an apparatus for generating a photomultipliercluster, said apparatus comprising:integration circuitry integratingsaid channel signals from said scintillation detector responsive to agamma event and generating integration results therefrom; peak circuitryfor determining a peak photomultiplier based on said integration resultsand for generating a signal indicative of said peak photomultiplier; andcluster circuitry addressed by said peak circuitry and responsive tosaid signal indicative of said peak photomultiplier for generating aphotomultiplier cluster associated with said peak photomultiplier, saidcluster circuitry generating separate photomultiplier clusters forindividual photomultipliers of said array of photomultipliers andwherein said cluster circuitry also generates a cluster type signalresponsive to said signal indicative of said peak photomultiplierwherein said cluster type signal indicates a geometric type of saidphotomultiplier cluster.
 2. An apparatus as described in claim 1 whereinsaid photomultiplier cluster comprises a set of photomultiplierindicators and further comprising:a sequencer circuit coupled to saidcluster circuitry, said sequencer circuit for addressing said clustercircuitry such that individual photomultiplier indicators of saidphotomultiplier cluster are sequentially output from said clustercircuitry; and weight circuitry responsive to (1) said photomultiplierindicators and (2) said cluster type signal, said weight circuitry forgenerating individual coordinate weight value signals associated withindividual photomultipliers of said photomultiplier cluster.
 3. Anapparatus as described in claim 2 wherein said photomultiplier clustercomprises a set of photomultiplier indicators and further comprising:abuffer storage for storing integrated results of said photomultipliercluster, said buffer storage addressed by photomultiplier indicatorsoutput from said cluster circuitry; and centroid computation circuitrycoupled to receive output integration results from said buffer storageand coupled to receive said individual coordinate weight value signalsassociated with individual photomultipliers of said photomultipliercluster, and in response thereto, said centroid computation circuitryfor generating a two dimensional spatial coordinate of said gamma event.4. An apparatus as described in claim 1 wherein said photomultipliercluster comprises a set of photomultiplier indicators and furthercomprising a sequencer circuit coupled to said cluster circuitry, saidsequencer circuit for addressing said cluster circuitry such thatindividual photomultiplier indicators of said photomultiplier clusterare sequentially output from said cluster circuitry.
 5. An apparatus asdescribed in claim 4 wherein said cluster circuitry comprises aprogrammable memory circuit addressed by said signal indicative of saidpeak photomultiplier and said sequencer circuit.
 6. An apparatus asdescribed in claim 1 wherein said cluster type signal indicates betweena normal cluster type, an edge cluster type and a corner cluster type.7. An apparatus as described in claim 1 further comprising a resolutionindicator signal coupled to address said cluster circuitry wherein saidphotomultiplier cluster that is output from said cluster circuitry isresponsive to said resolution indicator.
 8. An apparatus as described inclaim 7 wherein said cluster circuitry outputs photomultiplier clustersin sets of seven or less photomultipliers each, provided said resolutionindicator signal is set to low resolution and wherein said clustercircuitry outputs photomultiplier clusters in sets of more than sevenphotomultipliers each, provided said resolution indicator signal is setto high resolution.
 9. A gamma camera system comprising:a scintillationdetector for receiving gamma radiation, said scintillation detectorhaving an array of photomultipliers wherein individual photomultipliersgenerate channel signals; and an apparatus for processing said channelsignals, said apparatus comprising:integration circuitry integratingsaid channel signals responsive to a gamma event and generatingintegration results therefrom; peak circuitry for determining a peakphotomultiplier based on said integration results; cluster circuitryaddressed by said peak circuitry and responsive to a signal indicativeof said peak photomultiplier for generating addresses ofphotomultipliers of a photomultiplier cluster associated with said peakphotomultiplier, said cluster circuitry containing a listing ofphotomultiplier clusters associated with photomultipliers of said array;said cluster circuitry also for generating a type value signalindicative of a type of said photomultiplier cluster; and weightcircuitry responsive to (1) said addresses of photomultipliers withinsaid photomultiplier cluster and (2) said type value signal, said weightcircuitry for generating individual coordinate weight value signalsassociated with individual photomultipliers of said photomultipliercluster.
 10. A gamma camera system as described in claim 9 wherein saidcoordinate weight value signals associated with individualphotomultipliers of said photomultiplier cluster represent an X axisweight value and a Y axis weight value and wherein said weight circuitrycomprises:a first portion for containing weight value signals for eachphotomultiplier of said array for a first photomultiplier cluster type;and a second portion for containing weight value signals for eachphotomultiplier of said array for a second photomultiplier cluster type.11. A gamma camera system as described in claim 9 wherein said clustertype signal indicates a geometric configuration of said photomultipliercluster and includes a normal cluster type, an edge cluster type and acorner cluster type.
 12. A gamma camera system as described in claim 9further comprising:a buffer storage for storing integrated results ofsaid photomultiplier cluster, said buffer storage addressed byphotomultiplier addresses output from said cluster circuitry; andcentroid computation circuitry coupled to receive output integrationresults from said buffer storage and weight values from said weightcircuitry, said centroid computation circuitry for generating a twodimensional spatial coordinate of said gamma event.
 13. A gamma camerasystem comprising:a scintillation detector for receiving gammaradiation, said scintillation detector having a crystal and an array ofphotomultipliers optically coupled to said crystal wherein individualphotomultipliers generate channel signals in response to a gamma event;and process circuitry for processing said channel signals for generatinga spatial coordinate associated with said gamma event, said processcircuitry comprising:(a) integration circuitry integrating said channelsignals from said scintillation detector responsive to said gamma eventand generating integration results therefrom; (b) peak circuitry fordetermining a peak photomultiplier based on said integration results andgenerating a signal indicative of said peak photomultiplier; and (c)cluster circuitry addressed by said peak circuitry and responsive tosaid signal indicative of said peak photomultiplier for generating aphotomultiplier cluster associated with said peak photomultiplier, saidcluster circuitry containing an individual photomultiplier cluster forindividual photomultipliers of said array, wherein said clustercircuitry also generates a cluster type signal responsive to said signalindicative of said peak photomultiplier wherein said cluster type signalindicates a geometric type of said photomultiplier cluster including anormal cluster type, an edge cluster type and a corner cluster type. 14.A gamma camera system as described in claim 13, wherein saidphotomultiplier cluster comprises a set of photomultiplier indicatorsand further comprising a counter circuit coupled to said clustercircuitry, said counter circuit for addressing said cluster circuitrysuch that individual photomultiplier indicators of said photomultipliercluster are sequentially output from said cluster circuitry.
 15. A gammacamera system as described in claim 14 wherein said cluster circuitrycomprises a memory circuit addressed by said signal indicative of saidpeak photomultiplier and said counter circuit and further comprising:abuffer storage for storing integrated results of said photomultipliercluster, said buffer storage addressed by photomultiplier indicatorsoutput from said cluster circuitry; weight circuitry responsive to (1)said photomultiplier indicators and (2) said cluster type signal, saidweight circuitry for generating individual coordinate weight valuesignals associated with individual photomultipliers of saidphotomultiplier cluster; and centroid computation circuitry coupled toreceive output integration results from said buffer storage and coupledto receive said individual coordinate weight value signals, and inresponse thereto, said centroid computation circuitry for generating atwo dimensional spatial coordinate of said gamma event.
 16. A gammacamera system as described in claim 13 further comprising a resolutionindicator signal coupled to address said cluster circuitry wherein saidphotomultiplier cluster output from said cluster circuitry is responsiveto said resolution indicator and wherein said cluster circuitry outputsphotomultiplier clusters in sets of seven or less photomultipliers each,provided said resolution indicator signal is set to low resolution andwherein said cluster circuitry outputs photomultiplier clusters in setsof more than seven photomultipliers each, provided said resolutionindicator signal is set to high resolution.
 17. In a gamma camera systemhaving a scintillation detector for receiving gamma radiation, saidscintillation detector having an array of photomultipliers, a method ofdetermining a spatial coordinate of a gamma event, said methodcomprising the steps of:generating channel signals originating fromindividual photomultipliers of said array in response to said gammaevent; integrating said channel signals from said scintillation detectorto generate integration results therefrom; determining a peakphotomultiplier based on said integration results and generating asignal indicative of said peak photomultiplier; referencing a memorycircuit to generate a cluster type signal responsive to said signalindicative of said peak photomultiplier, said cluster type signalindicating a geometric type of said photomultiplier cluster including anormal cluster type, an edge cluster type and a corner cluster type;responsive to said signal indicative of said peak photomultiplier,referencing said memory circuit to generate a photomultiplier clusterassociated with said peak photomultiplier, said memory circuitcontaining an individual photomultiplier cluster for individualphotomultipliers of said array; and responsive to integration results ofphotomultipliers within said photomultiplier cluster and responsive tosaid cluster type signal, performing a centroid computation to determinesaid spatial coordinate of said gamma event.
 18. A method as describedin claim 17 wherein said photomultiplier cluster comprises a set ofphotomultiplier indicators and wherein said step of performing acentroid computation further comprises the steps of:sequentiallyaddressing said memory circuit with count values in addition to saidsignal indicative of said peak photomultiplier such that individualphotomultiplier indicators of said photomultiplier cluster aresequentially output from said memory circuit; and responsive to (1) saidindividual photomultiplier indicators and (2) said cluster type signal,generating individual coordinate weight value signals associated withindividual photomultipliers of said photomultiplier cluster.
 19. Amethod as described in claim 18 wherein said step of performing acentroid computation further comprises the step of generating a twodimensional spatial coordinate of said gamma event in response to saidindividual coordinate weight value signals and said integration resultsof photomultipliers within said photomultiplier cluster.
 20. A method asdescribed in claim 17 further comprising the step of providing aresolution indicator signal for addressing said memory circuit whereinsaid photomultiplier cluster output from said memory circuit isresponsive to said resolution indicator.
 21. A method as described inclaim 20 wherein said step of referencing a memory circuit to generate aphotomultiplier cluster associated with said peak photomultipliercomprises the step of:generating photomultiplier clusters in sets ofseven or less photomultipliers each, provided said resolution indicatorsignal is set to low resolution; and generating photomultiplier clustersin sets of more than seven photomultipliers each, provided saidresolution indicator signal is set to high resolution.
 22. In a gammacamera system having a scintillation detector for receiving gammaradiation, said scintillation detector having an array ofphotomultipliers wherein individual photomultipliers generate channelsignals, an apparatus for generating spatially dependent weight factors,said apparatus comprising:integration circuitry integrating said channelsignals from said scintillation detector responsive to a gamma event andgenerating integration results therefrom; cluster circuitry responsiveto said integration results for generating signals indicative ofphotomultipliers within a photomultiplier cluster defined by said gammaevent; circuitry for generating a type value signal indicative of a typeof said photomultiplier cluster; and weight circuitry responsive to (1)said signals indicative of photomultipliers within said photomultipliercluster and (2) said type value signal, for generating individualcoordinate weight value signals associated with individualphotomultipliers of said photomultiplier cluster.
 23. An apparatus asdescribed in claim 22 wherein said weight circuitry comprises aprogrammable memory device.
 24. An apparatus as described in claim 22wherein said weight circuitry comprises a static memory device.
 25. Anapparatus as described in claim 22 wherein said coordinate weight valuesignals associated with individual photomultipliers of saidphotomultiplier cluster represent an X axis weight value and a Y axisweight value.
 26. An apparatus as described in claim 25 wherein saidweight circuitry comprises a memory circuit and wherein said memorycircuit comprises:a first portion for containing weight value signalsfor each photomultiplier of said array for a first photomultipliercluster type; and a second portion for containing weight value signalsfor each photomultiplier of said array for a second photomultipliercluster type.
 27. An apparatus as described in claim 22 wherein saidcluster type signal indicates a geometric configuration of saidphotomultiplier cluster.
 28. An apparatus as described in claim 27wherein said cluster type signal indicates between a normal clustertype, an edge cluster type and a corner cluster type of saidphotomultiplier cluster.
 29. An apparatus as described in claim 22further comprising:a buffer storage for storing integrated results ofsaid photomultiplier cluster, said buffer storage addressed byphotomultiplier addresses output from said cluster circuitry; andcentroid computation circuitry coupled to receive output integrationresults from said buffer storage and weight values from said weightcircuitry, said centroid computation circuitry for generating a twodimensional spatial coordinate of said gamma event.
 30. In a gammacamera system having a scintillation detector for receiving gammaradiation, said scintillation detector having an array ofphotomultipliers wherein individual photomultipliers generate channelsignals, an apparatus for generating a photomultiplier cluster, saidapparatus comprising:integration circuitry integrating said channelsignals from said scintillation detector responsive to a gamma event andgenerating integration results therefrom; peak circuitry for determininga peak photomultiplier based on said integration results and forgenerating a signal indicative of said peak photomultiplier; aresolution mode signal indicative of a high or low resolution mode ofsaid gamma camera system; and cluster circuitry addressed by said peakcircuitry and by said resolution mode signal and responsive to saidsignal indicative of said peak photomultiplier and said resolution modesignal for generating (1) a photomultiplier cluster and (2) a clustertype signal, both associated with said peak photomultiplier, saidcluster circuitry containing an individual photomultiplier cluster andan individual cluster type for individual photomultipliers of saidphotomultiplier array.
 31. An apparatus as described in claim 30 whereinsaid cluster type signal indicates a geometric type of saidphotomultiplier cluster, wherein said geometric type is a normal clustertype, an edge cluster type or a corner cluster type, and furthercomprising a counter circuit coupled to said cluster circuitry, saidcounter circuit for addressing said cluster circuitry such thatindividual photomultiplier addresses of said photomultiplier cluster aresequentially output from said cluster circuitry.
 32. An apparatus asdescribed in claim 31 further comprising:a buffer storage for storingintegrated results of said photomultiplier cluster, said buffer storageaddressed by photomultiplier addresses output from said clustercircuitry; weight circuitry responsive to (1) said individualphotomultiplier addresses and (2) said cluster type signal, said weightcircuitry for generating individual coordinate weight value signalsassociated with individual photomultipliers of said photomultipliercluster; and centroid computation circuitry coupled to receive outputintegration results from said buffer storage and coupled to receive saidindividual coordinate weight value signals, and in response thereto,said centroid computation circuitry for generating a two dimensionalspatial coordinate of said gamma event.
 33. In a gamma camera systemhaving a scintillation detector for receiving gamma radiation, saidscintillation detector having an array of photomultipliers whereinindividual photomultipliers generate channel signals, a method forgenerating spatially dependent weight factors, said method comprisingthe steps of:integrating said channel signals from said scintillationdetector responsive to a gamma event and generating integration resultstherefrom; generating addresses indicative of photomultipliers within aphotomultiplier cluster defined by said gamma event; generating a typevalue signal indicative of a type of said photomultiplier cluster; andaccessing a memory circuit responsive to (1) said addresses indicativeof photomultipliers within said photomultiplier cluster and (2) saidtype value signal and generating in response thereto individualcoordinate weight value signals associated with individualphotomultipliers of said photomultiplier cluster.
 34. A method asdescribed in claim 33 further comprising the steps of:storing integratedresults of said photomultiplier cluster into a buffer storage, saidbuffer storage addressed by photomultiplier addresses output from saidcluster circuitry; and generating a two dimensional spatial coordinateof said gamma event responsive to (1) output integration results fromsaid buffer storage and (2) weight values from step of generatingindividual coordinate weight value signals associated with individualphotomultipliers of said photomultiplier cluster.
 35. A method asdescribed in claim 33 wherein said memory circuit is programmable andfurther comprising the step of programming said memory circuit withcoordinate weight value signals associated with photomultipliers of saidarray for different type value signals.
 36. A method as described inclaim 33 wherein said step of generating individual coordinate weightvalue signals associated with individual photomultipliers of saidphotomultiplier cluster comprises the step of generating an X axisweight value and a Y axis weight value for individual photomultipliersof said photomultiplier cluster.
 37. An apparatus as described in claim33 wherein said cluster type signal indicates a geometric configurationof said photomultiplier cluster, wherein said geometric configuration isone of a set of possible geometric configurations indictable by saidcluster type signal, said set of possible geometric configurationsincluding a normal cluster type, an edge cluster type and a cornercluster type.